U  G 


' 

OFFICE  OF  THE  CHIEF  SIGNAL  OFFICER,  1914 


RADIOTELEGRAPHY 


C-NRLF 


,  S.  SIGNAL  CORPS 


REVISED  MAY,  1915 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1915 


GIFT  OF 


CIRCULAR  NO.  1 
OFFICE  OF  THE  CHIEF  SIGNAL  OFFICER,  1914 


RADIOTELEGRAPHY 


U.  S.  SIGNAL  CORPS 

REVISED  MAY,  1915 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1915 


- 


TABLE  OF  CONTENTS. 


rage. 

Electric  charges  and  static  fields  of  force 5 

Forces  of  attraction  and  repulsion 5 

Currents  and  magnetic  fields  of  force 7 

Moving  charges  or  currents 7 

Direct  and  alternating  currents 7 

Static  and  magnetic  fields  near  a  wire 8 

Charges  with  static  lines .' . .  8 

Currents  with  magnetic  lines 8 

Radiation  of  electromagnetic  waves 9 

Velocity  of  propagation 9 

Currents  in  transmitting  and  receiving  antenna 9 

Measurement  of  potential  by  spark  discharge 10 

Needle  and  ball  spark  gaps 12 

Systems  of  units 13 

Electrostatic,  electromagnetic,  and  practical  systems 13 

Definitions  of  inductance  and  capacity 14 

Names  of  units 15 

Conversion  of  units  of  one  system  to  another 16 

Mechanical  and  electrical  oscillations 17 

Oscillatory  discharges;  wave  trains 17 

Damped  oscillations  with  spark  gap 17 

Undamped  oscillations  with  arc  and  high-frequency  alternator 19 

Frequency 19 

Resonance 20 

Power  circuits 21 

Transformers;  open  and  closed  magnetic  circuit  types;  oil  and  dry  insula- 
tion   21 

Alternators ;  revolving  field  and  armature  types ;  inductor  type 23 

Motor-generators 24 

Rheostat  and  reactance;  adjustment  of  power  circuits  by  reactance 24 

Key;  relay  and  "break"  type 25 

Definitions  of  alternating  current  terms - 26 

Frequency  and  period ;  frequency  meter 26 

Cycle  and  alternation 26 

Amplitude 27 

High-frequency  circuits 27 

Closed  oscillating  or  primary  circuit 27 

Essential  elements;  connections  to  transformer  secondary 27 

Duration  of  wave  train 28 

Uniform  spacing  of  wave  trains  and  purity  of  note 29 

Wave  train  or  spark  frequency;  relation  to  alternator  frequency .29 

Multiple  discharges 29 

Advantages  of  high-spark  frequency  at  transmitter 29 

Transmitting  condensers 30 

Function  and  types;*  brush  discharge  and  its  elimination;   series-parallel 

connection ;  capacity 30 

346958 

\ 


4  TABLE    OF   CONTENTS. 

Page. 

Transmitting  inductances 32 

Function  and  types,  calibiation  curves;  "skin''  effect;  change  of  resist- 
ance with  frequency  and  diameter  of  wire;  litzendraht  inductances 34-37 

Spark  g^aps • 38 

Function  and  types;    synchronous  and  nonsynchronous;   quenched  gap  and 

its  care 38-41 

Connection  of  closed  oscillating  circuit  to  antenna  circuit 41 

Plain  Marconi  antenna;    coupling,  direct  and  inductive;    close  and  loose 

coupling;  oscillation  transformer 42-45 

Antennae 45 

Types;  necessity  of  good  insulation ;  radiation  resistance ;  artificial  antenna; 

efficiency  of  radio  set 45-53 

Ground ;  necessity  of  surface  ground ;  counterpoise 53 

Wave  length  and  frequency *. 54 

Wave  meter;  indication  of  resonance  by  ammeter,  wattmeter,  detector,  etc. ; 

unipolar  detector  connection ;  fundamental  wave  length 55  56 

Tuning  of  transmitting  set 61 

Mechanical  illustration  of  coupling;  single  wave  length  with  loose  coupling; 
two  wave  lengths  with  close  coupling;  tuning  without  wave  meter  by 
maximum  antenna  current  or  potential;  tuning  with  wave  meter  to 
single  radiated  wave  length;  objection  to  transmitters  with  double  wave 

lengths 61-66 

Theory  of  quenched  spark  transmitter 66 

Opening  of  the  primary  circuit  by  quenching  or  stopping  of  spark;  advan- 
tages of  quenched  spark  transmitter;  test  of  proper  coupling  by  primary 

and  secondary  current. 66-68 

Receiving  circuits 68 

Direct  and  inductive  coupling;  untuned  and  tuned  secondary  circuits; 
changes  of  wave  length  with  changes  in  coupling;  changes  in  coupling 
with  changes  in  transmitter  damping;  elimination  of  static  and  interfer- 
ence; selective  circuits 68-76 

Detectors 76 

Coherer;  rectifiers;  audion;  advantages  of  high-spark  frequencies  at  re- 
ceiver   76-80 

Telephone  receivers 81 

High-resistance  windings;  with  adjustable  pole  pieces;  best  value  of  shunt- 
ing condenser;  group  tuning 81 

Calibration  of  receiving  circuits 82 

Use  of  buzzer  with  wave  meter  as  source  of  oscillations 82 

Signal  Corps  radio  equipment 82 

Fort  Sam  Houston  set;  1-kilowatt  Marconi  500-cycle  quenched  spark  sets 
for  Coast  Artillery  stations  with  instructions  for  installing  and  operating; 

Telef unken  field  wagon  set ;  Signal  Corps  field  pack  set 83-92 

Damping  and  measurement  of  logarithmic  decrement 122 

Definitions;  use    of    decremeter    and    wave    meter;  formulas;  resonance 

curve  for  computation  of  logarithmic  decrement 122-128 


CIRCULAR  No.  1. 


RADIOTELEGRAPHY. 


ELECTRIC    CHARGES    AND    STATIC    FIELDS    OF   FORCE. 

Electrical  phenomena  may  be  grouped  under  two  general  classes, 
those  of  static  electricity,  when  the  electrical  charges  are  at  rest,  and 
dynamic  or  current  electricity,  when  the  charges  are  in  motion  along 
a  conductor. 

When  an  insulator,  such  as  sealing  wax,  is  rubbed  with  fur,  or  a 
glass  tube  with  silk,  it  acquires  the  property  of  attracting  light  bodies 
near  it,  and  is  said  to  be  charged.  This  action  shows  that  forces  exist 
in  the  adjacent  space,  and  there  is  said  to  be  an  electrostatic,  or,  more 
briefly,  a  static  field  of  force  about  the  charged  body.  When  two 
charged  bodies  are  brought  near  together,  they  may  be  either  attracted 


FIG.  1. 

or  repelled,  depending  on  the  nature  of  the  two  charges.  If  the 
rubbed  glass  or  particles  touched  and  thereby  charged  by  it  are 
brought  near  the  sealing  wax  or  particles  charged  by  it,  they  will 
attract  each  other,  but  two  bodies  both  of  which  have  been  pre- 
viously charged  by  either  the  glass  or  the  wax  will  repel  each  other. 
Hence  like  charges  repel  and  unlike  charges  attract.  The  names 
positive  (glass)  and  negative  (sealing  wax)  have  been  given,  re- 
spectively, to  these  charges.  By  means  of  a  delicately  suspended 
insulated  body  we  can  map  out  the  forces  along  directions  in  general 
perpendicular  to  the  charged  surfaces.  In  figure  1  is  shown  in  sec- 
tion the  static  field  of  force  between  a  positively  charged  and  a  nega- 
tively charged  body  in  which  the  direction  of  the  field  at  any  point 

5 


BADIOTELEGRAPHY. 


is  indicated  by  the  direction  of  the  arrows  at  that  point,  and  the 
intensity  or  strength  of  the  field  in  any  area  is  indicated  by  the 
number  of  lines  in  that  area.  It  is  seen  that  most  of  the  lines  are 


FIG.  2. 


crowded  together  between  the  two  as  though  there  was  an  actual  pull 
along  their  length,  thus  suggesting  attraction.  Similarly  in  figure 
2  are  shown  the  static  lines  between  two  bodies  with  positive  charges 
which  are  apparently  driven  apart,  thus  suggesting  repulsion.  If 


FIG.  3. 


both  charges  were  negative  the  direction  of  the  arrows  would  have 
to  be  reversed,  but  the  static  lines  would  have  the  same  shape  as 
before.  In  figure  3  are  shown  in  elevation  the  static  lines  from  a 


RADIOTELEGKAPHY.  7 

positively  "'charged  wire  near  the  surface  of  the  earth.  If  the  wire 
were  negatively  charged  the  signs  of  the  charges  and  the  direction  of 
the  arrows  would  have  to  be  reversed. 

CURRENTS   AND   MAGNETIC   FIELDS   OF   FORCE. 

If  a  wire  connects'a  charged  body  with  an  uncharged  or  oppositely 
charged  one,  the  static  charge  will  flow  through  the  wire  from  the 
charged  to  the  uncharged  body,  or  from  the  positively  charged  body 
to  the  negatively  charged  one,  and  become  a  current  while  so  flowing  • 


FIG.  4. 

that  is,  a  current  is  a  moving  charge  or  succession  of  charges.  If  the 
same  charge  is  continuously  renewed  we  have  a  steady  or  direct  cur- 
rent, often  abbreviated  as  D.  C.  If  the  charges  are  continuously  vary- 
ing in  intensity  and  sign  and  the  variations  are  periodic  in  character, 
we  have  an  alternating  current,  or  A.  C. 

While  the  current  is  flowing  in  the  wire  we  find  that  there  exists 
around  it  a  field  of  force  of  another  kind.  If  a  horizontal  magnetic 
needle  is  brought  near  a  vertical  wire  in  which  a  direct  current  is 
flowing,  the  needle  will  be  deflected  and  the  direction  in  which  it  will 
point  depends  upon  the  direction  in  which  the  current  is  flowing. 


8 


EADIOTELEGEAPHY. 


This  action  shows  that  forces  exist  in  the  adjacent  space,  and  the  wire 
carrying  the  current  is  said  to  have  a  magnetic  field  about  it.  The 
lines  of  magnetic  force  may  be  mapped  out  with  iron  filings  or  a  mag- 
netic compass.  Thus,  if  the  compass  is  moved  in  the  direction  indi- 
cated by  the  deflection  of  its  needle  it  will  trace  out  circles  around 
the  wire  as  a  center  and  in  planes  perpendicular  to  it. 

Thus,  in  figure  4  is  shown  a  section  of  a  wire,  perpendicular  to  the 
paper  and  carrying  a  current  downward  through  it,  surrounded  by 
circles,  which  by  the  direction  of  the  arrows  indicate  the  direction  of 
the  magnetic  field  at  any  point,  and  by  the  number  of  lines  in  any 
area  indicate  the  intensity  of  the  magnetic  field  in  that  area.  If  the 
direction  of  the  current  in  the  wire  were  reversed  so  as  to  flow  up 
through  the  paper,  the  direction  of  the  arrows  would  have  to  be  re- 


FlG.  5. 

versed.  Similarly,  in  figure  5  the  wire  is  shown  lying  on  the  paper 
and  the  current  flowing  toward  the  top  of  the  page,  with  the  mag- 
netic lines  (appearing  as  dots)  going  down  through  the  paper  on  the 
right  of  the  wire  and  coming  up  through  on  the  left. 

STATIC   AND   MAGNETIC   FIELDS   NEAR   A  WIRE. 

If  a  long  wire  is  placed  vertically,  and  positive  and  negative  charges 
are  alternately  applied  at  the  bottom  and  flow  along  the  wire,  we 
shall  have  near  the  wire  alternately  opposite  static  fields,  due  to  the 
charges;  and  at  the  same  time  alternately  opposite  magnetic  fields, 
due  to  the  alternating  currents.  Thus,  figure  6  shows  in  perspective 
the  wire  with  a  positive  charge,  surrounded  by  its  vertical  static  field 
S  and  its  horizontal  magnetic  field  M,  and  figure  7  the  wire  with  a 
negative  charge  and  both  its  fields  reversed  in  direction.  Both  the 


RADIOTELEGRAPHY. 


9 


static  and  magnetic  lines  as  seen  when  projected  on  the  plane  below 
the  wire  are  shown  together  in  figure  8,  where  the  magnetic  lines  are 
circles,  as  in  figure  4,  and  the  static  lines  are  straight,  being  radial 
with  respect  to  the  circles. 

RADIATION   OF   ELECTROMAGNETIC   WAVES. 

These  two  fields  of  force  changing  their  direction  and  intensity  with 
great  rapidity  and  traveling  outward  from  the  wire  in  the  medium 
called  the  ether  with  the  velocity  of  light,  300,000,000  meters  or 
186,000  miles  per  second,  are  the  electromagnetic  waves  of  radio- 


M 


FIG.  6. 

telegraphy.  They  spread  simultaneously  radially  outward  and  up- 
ward from  the  antenna,  as  this  vertical  wire  is  called.  The  energy 
of  the  varying  electric  charges  and  currents  is  thus  imparted  to  the 
medium,  or  is  radiated. 

The  two  fields  constituting  the  wave  and  their  outward  motion  in 
radiation  are  shown  in  a  general  way  in  figure  9,  where  the  electric 
field  is  indicated  as  lines  and  the  magnetic  field  as  dots,  this  latter 
being  necessary,  as  in  figure  5,  because  the  magnetic  field  is  perpen- 
dicular to  the  plane  of  the  paper.  At  great  distances  from  the  trans- 
mitting antenna  the  static  lines  become  straight  and  perpendicular 


10 


KADIOTELEGRAPHY. 


to  the  surface  of  the  earth  and  the  magnetic  lines  straight  and  parallel 
to  the  surface. 

These  static  and  magnetic  lines  of  force,  moving  with  the  velocity 
of  light,  sweep  across  the  antenna  at  the  receiving  station.  The 
vertical  static  lines  hi  the  wave  are  directed  alternately  upward  and 
downward  and  produce  in  the  antenna  moving  charges  of  alternately 
opposite  signs;  that  is,  an  alternating  current.  At  the  same  tune 
the  horizontal  magnetic  lines  are  directed  alternately  to  the  right  and 
left,  and  when  cutting  across  the  antenna  produce  an  alternating 


FIG.  7. 

current  in  it.  The  resultant  current  generated  by  these  two  fields 
gives  an  alternating  current  in  the  receiving  antenna  quite  similar  to 
that  hi  the  transmitting  antenna,  although  of  course  much  weaker. 
It  is  these  alternating  currents  which  produce  the  signals  in  the 
receiving  apparatus. 

MEASUREMENT   OF   POTENTIAL  BY   SPARK  DISCHARGE. 

If  large  charges  of  opposite  signs  are  given  to  two  insulated  bodies 
close  together,  a  spark  will  jump  between  them  and  the  potential  is 
said  to  be  high.  The  distance  between  the  points  of  two  needles 
mounted  in  the  same  line  may  be  used  to  measure  this  potential.  The 


KADIOTELEGEAPHY. 


11 


FIG.  8. 


|< Ware  length 


Fro.  9. 


12  RADIOTELEGRAPH  Y. 

distance  between  two  brass  balls  each  2  centimeters  (about  25/32 
inch)  in  diameter  may  also  be  used.  It  will  be  found  that  the  needle 
points  are  more  useful  at  low  voltages,  as  from  5,000  to  15,000,  and 
the  brass  balls  more  useful  at  the  higher  values.  In  figures  10  and  11 
are  given  the  voltage  curves  for  the  needle  and  the  ball  gaps.  Thus, 
if  the  discharge  occurs  between  needle  points  one-half  of  an  inch 
apart  the  potential  is  15,000  volts.  In  Tables  1  and  2  are  given  the 
values  from  which  the  curves  are  plotted. 

TABLE  1. — Needle  points. 

[Adapted  from  the  table  of  the  American  Institute  of  Electrical  Engineers.] 

Sparking  distance  -    Maximum  potential 

in  inches.  in  volts. 

0.  15 5,  000 

.20 6,  400 

.  30  9,  300 

.  40  12,  200 

.50 15,  000 

.  60  17,  700 

.  70  20,  500 

.  80  23, 100 

0.  90  25,  700 

1.  00 28,  300 

.  10  30,  700 

.  20  33,  000 

.  .  30  35,  300 

.  40  37,  500 

.50 . 39,  700 

.  60  41,  900 

.  70  43,  900 

.  80  45,  800 

1.  90  47,  600 

2.  00  49,  500 

The  potential  is  the  maximum  or  peak  value  of  the  wave 

TABLE  2. — Brass  balls  2  centimeters  in  diameter. 
[Adapted  from  Prof.  Fleming's  book  "The  Principles  of  Electric  Wave  Telegraphy."] 

Sparking  distance  Maximum  potential 

in  inches  in  volts. 

0.  05 5,  700 

.10 10,  000 

.20 17,  700 

.30 25,  000 

.40 31,  700 

.  50 36,  700 

.60.... 40,600 

.70 44,  300 

.80 47,  700 

.90 50,  800 

1.  00 53,  400 

The  potential  is  the  maximum  or  peak  value  of  the  wave. 


EADIOTELEGEAPHY. 
SYSTEMS   OF   UNITS. 


13 


Inductances  and  capacities  are  essential  elements  in  the  circuits 
for  generating  and  detecting  electromagnetic  waves.  Their  defini- 
tions and  the  units  in  which  they  are  measured  will  be  briefly  given 
in  the  following  paragraphs. 

A  condenser  is  said  to  have  capacity,  which  may  be  defined  as  its 
property  of  storing  the  energy  of  electric  charges  in  the  form  of  an 
electrostatic  field,  as  mentioned  on  page  9. 


SPARKING  DISTANCE  BE 

TWEEN  NEEDLE   POINTS 

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Efl  liiiiiiiilili      ii  •  i'lliiiiiiliiiiiiliiliiili  i 

Bw     fl::::i:::i::i       :i:i::\:J:::i:::::ii:i:iii::iii  i 

mm  liiiiiiiii  i  iiililili              iilliii 

K^^^QE:  ::::;:  I  i  ::!:::::::    E  i:      :::::::::::::::::; 

::::::::::::::::::  :  ::  ::::::::::::::::::::::::::::::! 

••••••••••••SSSSSS  •  ••    S!!SSSSSS!SS!S!!S!SSSSSSS!SSSS^I 
•••••••••••••••••••••••••••••••••••••••I  !••••••••••••••  ^H 

""!  """"-""""v::::::  ::::::::::::::::::::::l 
ii::  i::::i::i::iii:::i:::ii::  :::::::::::::::::::::• 

•SSSSSSSSS""!"!!!!!"!"        SSSS!S!S!SSSSSSSSSSS!SH 

••••••••••"••••••••••••••a        ••••••••••••••••••••••^H 

ililiiiiliiiiiiiiiiiiiiii::   :iii[iiiiiii:EiiEEiii[:l 

••••  •••••  ••  •••  ••••••!••      •••••••••••••••••••••••^H 

illii  11  J  i  ill  iiliillii  llllliyiiiiiiiiiEEliliJl 

0      .10    .20          .40           .60           .80            LOO          1.20          1-40          1.60          1-80         ZOO 
Sparking  Distance  in  Inches 

FIG.  10. 

A  coil  is  said  to  have  inductance^  which  may  be  defined  as  its 
property  of  storing  the  energy  of  electric  currents  in  the  form  of  a 
magnetic  field,  as  mentioned  on  page  9. 

Capacity  and  inductance,  as  well  as  the  other  electrical  quantities, 
can  be  measured  in  three  different  systems  of  units,  the  electrostatic, 
electromagnetic,  and  practical.  From  some  points  of  view  it  is  un- 
fortunate that  three  different  systems  have  come  into  general  use, 


14 


BADIOTELEGRAPHY. 


but  it  is  now  impossible  to  abandon  any  one  of  them.     The  rela- 
tions between  the  systems  may  be  briefly  explained  as  follows: 

The  units  of  the  electrostatic  system  may  be  considered  as  based 
on  the  value  of  a  unit  quantity  or  charge  of  electricity  such  that  if 
two  bodies  are  charged  with  it  they  will  repel  each  other  with  a 
unit  force  when  placed  at  a  unit  distance  apart.  If  this  charge 
flows  along  a  wire  it  becomes  a  current,  and  if  the  unit  charges 
are  renewed  at  the  rate  of  one  every  second  the  current  so  obtained 


50,000 


45,000 


40,000 


35,000 


30,000 


25,000 


SPARKING  DISTANCE  BETWEEN  BALLS  2  CM.  IN  DIAMETER 


^20,000 


15,000 


10,000 


5,000 


.10          .2p         -30         .40 

Sparking  Distance  in  inches 


.70 


.80 


.90 


1.00 


FIG.  11. 


is  called  a  unit  current  in  the  electrostatic  system.  The  units  of  the 
electromagnetic  system  may  be  considered  as  based  on  the  value 
of  a  unit  current  of  electricity  such  that  its  magnetic  field  will  exert 
the  same  unit  force  as  mentioned  above  on  a  body  with  a  unit 
magnetic  field  when  placed  at  a  unit  distance  from  a  unit  length  of 
wire  carrying  this  current.  The  current  so  defined  is  called  the  unit 
current  in  the  electromagnetic  system. 

The  strength  or  intensity  of  these  two  unit  currents  is  not  the 
same;  in  fact,  it  is  very  different,  that  of  the  current  in  the  electro- 


RADIOTELEGRAPH  Y.  15 

magnetic  system  being  30,000,000,000  times  stronger  than  the  unit 
current  in  the  electrostatic  system.  The  units  of  the  other  electrical 
quantities,  as  capacity,  inductance,  resistance,  etc.,  are  likewise  nearly 
all  different  in  the  two  systems,  in  some  cases  the  units  being  larger 
in  one  system  than  in  the  other,  and  vice  versa.  Owing  to  the  incon- 
venient size  of  the  units  in  the  two  previous  systems,  suitable  frac- 
tions or  multiples  of  these  units  have  been  chosen  as  the  units  of  the 
practical  system.  The  numerical  relations  between  the  units  of  the 
three  systems  are  given  in  textbooks,  so  that  only  a  few  of  the  more 
useful  ones  will  be  included  in  the  table  below. 

It  is  sometimes  convenient  to  abbreviate  the  words  "electrostatic" 
and  "electromagnetic"  to  "static"  and  "magnetic,"  as  has  been  done 
in  the  table  on  the  next  page,  and  also  to  write  more  shortly  E.  S. 
and  E.  M. 

When  capacity  is  measured  in  the  practical  system  the  units  are 
the  farad  and  the  one-millionth  part  of  a  farad,  called  the  microfarad, 
and  in  the  electrostatic  system  the  unit  is  the  centimeter.  The  relation 
between  the  two  as  shown  in  the  table  is  as  follows : 

Number  of  static  units  or  centimeters  „  .     ,       . 

.  -  =  number  of  practical  units  or 

yuu,uuu 

microfarads;  thus, 

1,000  cms.-ggOgmfd.-^  mfd.-O.OOlll  mid. 

Similarly  900,000  X  number  of  microfarads  =  number  of  centimeters. 

The  unit  of  capacity  in  the  electromagnetic  system  has  received 
no  name,  but  if  measured  in  this  system  the  units  can  be  converted 
into  those  of  the  other  systems  by  means  of  the  table. 

When  inductance  is  measured  in  the  practical  system  the  unit  is 
the  henry  with  its  fractional  parts,  as  the  one-thousandth  part,  called 
the  millihenry,  and  the  one-millionth  part,  called  the  microhenry. 
Thus,  1/1,000  henry  =1  millihenry,  and  1/1,000,000  henry  =  1  micro- 
henry; 1  henry  =1, 000  millihenrys  =  1,000,000  microhenrys.  In  the 
electromagnetic  system  the  unit  of  inductance  is  the  centimeter.  It 
is  to  be  noted  that  the  name  of  this  unit  is  the  same  as  that  of  the 
unit  of  capacity  in  the  electrostatic  system.  The  relation  between 
the  units  of  inductance  of  the  two  systems  is  as  follows : 
Number  of  magnetic  units  or  centimeters  »  , .  ,  ., 

1,000,000,OOT"  =number  of  practical  units, 

or  henrys;  and  similarly  1,000,000,000  X  number  of  henrys  =  num- 
ber of  centimeters;  1,000  cms.  =  1  microhenry  =1/1, 000, 000  henry  = 
.000,001  henry;  1,000,000  cms.  =  l  millihenry  =  1/1,000  henry  =.001 
henry;  1,000,000,000  cms.  =  1  henry.  Thus 

566  henrv  =  ljm  henrv  =  °-002  henrv' 

=  .002  X  1,000,000  microhenrys  =  2,000  microhenrys. 
=  .002  X  1,000  millihenrys  =  2  millihenrys. 
=  .002  X  1,000,000,000  cms.=  2,000,000  cms. 


16 


RADIOTELEGRAPHY. 


The  unit  of  inductance  in  the  electrostatic  system  has  received  no 
name  but  can  be  converted  into  units  of  the  other  systems  by  the 
table. 

Table  for  changing  some  of  the  more  common  units  from  one  system  to  another. 

CAPACITY. 


Electrostatic  units  (in  cms.). 


To  magnetic.     ;     To  practical. 


Electromagnetic  units  (no  name),  j       Practical  units  (in  mfd.). 


To  static. 


To  practical. 


To  static.       |   To  magnetic. 


Divide  by 


9X1020 


Divide  by 


900,000 


Multiply  by 


9X1020  ; 


Multiply  by 


Multiply  by 
900,000 


Divide  by 


INDUCTANCE. 


Electrostatic  units  (no  name). 

Electromagnetic  units  (cms.). 

Practical  units  (in  henrys). 

To  magnetic. 

To  practical. 

To  static. 

To  practical. 

To  static. 

To  magnetic. 

Multiply  by 
9X1020 

Multiply  by 
9X10" 

Divide  by 
9X1020 

Divide  by 
1X109 

Divide  by 
9X10" 

Multiply  by 
1X109 

CUHRENT. 


Electrostatic  units  (no  name). 

Electromagnetic  units  (no  name). 

Practical  units  (in  amperes). 

To  magnetic. 

To  practical. 

To  static. 

To  practical. 

To  static. 

To  magnetic. 

Divide  bv 
"3X1010 

Divide  by 
3X109 

Multiply  by 
3X10") 

Multiply  by 
10 

Multiply  by 
3X109 

Divide  bv 
10 

POTENTIAL. 


Electrostatic  units  (no  name). 

Electromagnetic  units  (no  name). 

Practical  units  (in  volts). 

To  magnetic. 

To  practical. 

To  static. 

To  practical. 

To  static. 

To  magnetic. 

Multiply  by 
3X10W 

Multiply  by 
300 

Divide  by 
3X10io 

Divide  by 
1X108 

Divide  by 
300 

Multiply  by 
1X108 

KESISTANCE. 


Electrostatic  units  (no  name). 

Electromagnetic  units  (no  name). 

Practical  units  (in  ohms). 

To  magnetic. 

To  practical. 

To  static. 

To  practical. 

To  static.         To  magnetic. 

| 

Multiply  by 
9X1020 

Multiply  by 
9X10" 

Divide  by 
9X1020 

Divide  by 
IXlQs 

. 

Divide  by         j  Multiply  by 
9X10"                     1X109 

It  will  be  noted  that  in  many  cases  the  units  have  received  no  name 
in  some  of  the  systems  in  which  they  are  expressed,  so  that  the  name 
of  the  system  must  be  given;  thus  a  current  of  1  ampere  is  a  cur- 
rent of  3,000,000,000  units  of  current  in  the  electrostatic  system,  or 
3,000,000,000  electrostatic  units  of  current. 


RADIOTELEGRAPHY.  17 

Owing  to  the  large  numbers  which  must  be  used  in  converting  units 
from  one  system  to  another  it  is  usual  to  abbreviate  as  in  algebra; 
thus,  3,000,000,000  is  written  3  X  109,  where  the  number  9  indicates 
the  number  of  times  that  the  cipher  or  zero  must  be  written  after  the 
number  3,  and  similarly  900,000,000,000,000,000,000  is  written  9  X  1020. 

The  table  may  be  used  to  convert  from  one  system  to  another,  as 
follows:  A  potential  of  2.5  units  in  the  E.  S.  system  is  equal  to 
2.5X300  units  in  the  practical  system,  or  750  volts;  current  of  1.0 
ampere  in  the  practical  system  is  equal  to  1.0 -r- 10  units  of  current  in 
the  E.  M.  system,  or  0.1  unit  in  the  E.  M.  system;  an  inductance  of 
1/500  henry  is  equal  to  1/500  X  109  E.  M.  units  of  inductance  or  centi- 
meters, or  1/500x1,000,000,000  =  2,000,000  cms. 

MECHANICAL   AND   ELECTRICAL   OSCILLATIONS. 

The  following  illustrations  and  explanations  of  oscillatory  dis- 
charges and  their  occurrence  in  resonant  circuits  are  introduced  here 
so  as  to  give  a  clear  understanding  of  these  most  important  principles. 

OSCILLATORY   DISCHARGES. 

If  a  strip  of  steel  is  clamped  at  one  end  and  the  free  end  is  pulled 
to  one  side  and  released,  this  end  will  not  only  return  to  its  normal 
position  but  will  swing  past  it,  and  returning  it  will  execute  a  series 
of  oscillations,  each  of  which  takes  place  in  the  same  length  of  time 
expressed  in  fractions  of  a  second,  which  will  gradually  die  down 
to  zero,  or  are  said  to  be  damped.  The  free  end  returns  to  its  posi- 
tion because  of  the  elasticity  of  the  metal,  and  swings  beyond  its 
normal  position  because  of  its  inertia.  The  energy  stored  up  in  the 
spring  in  pulling  it  to  one  side  is  thus  gradually  wasted  in  friction, 
etc.  In  a  similar  way  in  electrical  circuits  we  have  to  deal  with 
capacity,  which  corresponds  to  the  elasticity,  and  inductance,  which 
corresponds  to  the  inertia. 

If  a  condenser  of  considerable  capacity  C,  such  as  a  number  of 
Ley  den  jars  or  condenser  plates  in  parallel,  is  connected  in  a  circuit 
with  a  coil  L  and  spark  gap  S,  as  shown  in  figure  12,  and  the  poten- 
tial on  the  condenser  gradually  increased,  quite  a  large  charge  may  be 
stored  in  it  before  the  potential  rises  high  enough  to  cause  a  spark  at 
the  gap.  When,  however,  the  gap  breaks  down,  the  charge  in  the 
condenser  discharges  through  the  gap  and  the  coil,  and  on  account 
of  the  inductance  (inertia)  in  the  circuit  it  overshoots  in  the  same  way 
as  the  spring,  then  discharges  in  the  opposite  direction,  etc.,  so  that 
the  charge  may  oscillate  many  times  back  and  forth  across  the  gap 
before  it  is  so  used  up  in  heat  that  not  enough  charge  remains  to 

92448—15 2 


18 


KADIOTELEGRAPHY. 


jump  across  again.  The  charged  condenser,  as  C  of  figures  12  and  17, 
is  thus  the  immediate  source  of  the  energy  of  the  electrical  oscillations. 
Its  rapid  oscillatory  discharge  through  the  gap  S  and  the  induct- 


oso 


FIG.  12. 


ance  L  takes  place  in  the  form  of  a  series  of  decreasing  oscillations, 
called  a  train  of  damped  oscillations  or  a  damped  wave  train.     In 


second 


FIG.  13. 


some  circuits  there  may  be  20,  30,  or  even  more  such  oscillations  in  a 
wave  train.     Figure  13  represents  discharges  hi  which  the  oscillations 


FIG.  14. 


die  down  quickly,  and  are  said  to  be  strongly  damped  or  highly 
damped.  Figure  14  represents  discharges  in  which  the  oscilla- 
tions die  down  gradually  and  are  said  to  be  feebly  damped  or  sligJitly 


RADIOTELEGRAPH  Y.  19 

damped.  Figure  1 5  represents  discharges  in  which  the  oscillations 
do  not  die  down  and  are  said  to  be  undamped  oscillations,  con- 
tinuous oscillations,  or  sustained  oscillations.  These  undamped  oscil- 
lations can  not  be  generated  by  the  discharge  of  a  condenser  through 
an  ordinary  spark  gap,  but  may  be  developed  by  means  of  a  special 
type  of  direct-current  arc  with  metal  or  metal  and  carbon  electrodes, 
as  in  the  Poulsen  or  Federal  system,  or  by  special  high-frequency 
alternators,  as  in  the  Fessenden  or  Goldschmidt  system.  One  of 
these  alternators  having  a  speed  of  20,000  revolutions  per  minute 
and  giving  100,000  oscillations  per  second  has  been  installed  by  the 
Signal  Corps  at  the  Bureau  of  Standards  in  Washington,  D.  C.  This 
machine  and  its  driving  motor  are  shown  in  figure  16.  Both  the  arc 
and  alternator  methods  of  the  generation  of  undamped  oscillations 
are  now  in  use. 


FIG.  15. 
,  FREQUENCY. 

The  rate  of  vibration  of  the  steel  spring  or  number  of  vibrations 
per  second  depends  upon  the  weight,  distribution,  and  elasticity  of 
the  metal.  Similarly  in  the  electrical  circuit,  when  the  condenser 
discharges  across  the  gap  and  through  the  inductance,  the  rate  of 
the  electrical  oscillations,  or  frequency  in  number  of  oscillations  per 
second,  depends  upon  the  capacity  of  the  condenser  and  the  induc- 
tance of  the  coil.  The  larger  the  product  of  the  capacity  and  induc- 
tance, the  slower  is  the  rate  of  the  oscillations;  that  is,  the  fewer  the 
number  of  oscillations  per  second  and  the  lower  the  frequency,  and 
vice  versa,  the  smaller  the  product  of  the  capacity  and  inductance  the 
more  rapid  is  the  rate  of  the  oscillations;  that  is,  the  greater  the 
number  of  oscillations  per  second  and  the  higher  the  frequency.  The 

formula  for  the  number  of  oscillations  per  second  is  n  =  = — 7^=  where 

L  is  the  inductance  in  circuit  in  henrys  and  C  the  capacity  in  farads ; 
thus,  if  C  is  0.000,000,004  farad  (0.004  microfarad)  and  L  is  0.001 
henry  (1,000,000  cms.  or  1  millihenry),  then  the  oscillations  are  tak- 
ing place  at  the  rate  of  about  79,600  per  second. 

J !__. 

~~  6.28N/0.001  X  0.000,000,004  =  6.28N/o.OOO,000,OOO^J§4  =  79)  6°C 


20 


KADIOTELEGRAPHY 
RESONANCE. 


The  principles  of  resonance  can  be  illustrated  by  the  steel  spring, 
preferably  in  the  form  of  two  tuning  forks.  If  a  loud  note  from 
one  tuning  fork  is  sounded  near  another  fork,  the  latter  will  be  set 
in  vibration  slightly,  even  if  the  pitch  of  the  note  or  number  of 
vibrations  per  second  is  not  the  same  as  that  which  the  latter  itself 


would  give.  If,  however,  the  note  is  of  the  same  pitch,  then  each 
successive  vibration  of  the  prongs  will  be  reenforced  by  air  waves 
of  the  same  frequency  as  its  own,  and  stronger  vibrations  will  be 
produced  by  this  note  than  by  any  other.  Under  these  conditions 
the  two  forks  are  said  to  be  in  resonance.  Similarly  if  a  circuit 
containing  a  coil  1,  condenser  c,  and  very  small  spark  gap  s,  all  in 


EADIOTELEGRAPHY.  21 

series,  is  brought  near  another  circuit  LCS,  as  shown  in  figure  17, 
in  which  oscillations  are  taking  place,  then  small  sparks  may  be 
seen  passing  across  the  gap  s,  of  the  first  circuit,  showing  that  cur- 
rents are  being  induced  in  it.  If,  however,  adjustments  are  made 
in  the  number  of  the  Leyden  tubes  in  circuit  or  in  the  number  of 
turns  of  inductance  by  means  of  the  sliding  contact,  then  generally 
the  size  and  brightness  of  the  spaiks  will  be  increased  up  to  a  certain 
point,  and  any  further  changes  in  either  the  inductance  or  the  ca- 
pacity will  make  the  spaiks  smaller  and  fainter.  At  the  adjust- 
ment which  gives  the  largest  and  brightest  sparks  the  induced  oscil- 


FIG.  17. 

lations  are  the  strongest  and  of  the  same  frequency  in  the  two  cir- 
cuits; that  is,  the  two  circuits  are  syntonized,  or  tuned,  or  are  in 

resonance. 

POWER   CIRCUITS. 

TRANSFORMERS. 

After  each  oscillatory  discharge  the  charge  in  the  condenser  is 
renewed  at  regular  intervals  by  an  induction  coil,  or  alternating  cur- 
rent transformer.  The  former  is  but  little  used  now,  and  will  not 
be  described  here.  The  transformer  is  an  apparatus  for  increasing 
the  comparatively  low  voltage  of  an  alternating  current  dynamo  or 
generator  to  the  high  voltage  necessary  to  cause  the  condenser 
charge  to  jump  across  the  spark  gap.  The  details  of  transformer 
construction  are  desciibed  in  textbooks  on  electricity,  and  it  will 
suffice  to  say  here  that  it  consists  of  a  primary  winding  of  a  com- 
paratively few  turns  of  heavy  wire,  wound  on  laminated  iron  or 
iron-wire  core,  which  carries  the  current  from  the  alternator,  and  a 
secondary  winding  of  many  turns  of  finer  wire,  well  insulated  from 
all  other  parts  of  the  transformer,  which  delivers  a  smaller  current, 
but  at  the  necessarily  higher  voltage,  to  the  condenser  that  is  charged 
thereby.  In  general  the  transformer  increases  the  alternator  or 
primary  voltage  in  the  same  proportion  as  the  number  of  secondary 
turns  is  increased  over  the  number  of  the  primary  turns.  The 
voltage  of  the  alternator  impressed  on  the  primary  of  the  trans- 
former is  usually  110  or  220  volts;  the  voltage  of  the  secondary 
which  is  impressed  on  the  condenser  depends  upon  the  size  of  the 
radio  set  and  varies  between,  say,  10,000  and  30,000  volts. 


22 


EADIOTELEGEAPHY. 


In  the  case  of  quenched  spark  sets  a  transformer  is  generally  used  in 
which  by  a  proper  choice  of  the  capacity  connected  to  its  secondary 
circuit,  the  secondary  voltage  is  increased  by  resonance  to  perhaps 
twice  as  many  tunes  as  the  ratio  of  the  primary  and  secondary  turns 
would  indicate.  Such  a  transformer  is  called  a  resonance  transformer. 
Transformers  may  be  divided  into  two  classes,  depending  on  the 
type  of  the  laminated  core,  whether  with  the  open  magnetic  circuit, 

as  shown  in  figure  18,  or 
with  the  closed  magnetic 
circuit,  as  shown  in  figure 
19.  These  terms  apply  to 
the  iron  as  a  path  for  the 
magnetic  field.  Thus  in 
figure  19  it  is  seen  that  the 
magnetic  lines  M  have  a 
continuous  path  or  circuit 
through  the  iron,  or,  as  it  is 
said,  a  closed  magnetic  cir- 
cuit whereas  in  figure  18 
the  path  of  the  lines  is 
partly  through  the  iron  and 
partly  through  the  space 
outside,  or,  as  it  is  said,  an 
open  magnetic  circuit.  In  both  figures  the  direction  of  the  field  as 
it  exists  at  one  instant  is  indicated  by  arrows,  but  it  must  be  remem- 
bered that  the  field  is  continually  reversing  its  direction  as  the  alter- 
nating current  changes  its  direction.  Both  types  of  transformers  are 
in  general  use  and  there  is  no  essential  difference  in  their  efficiency. 
Practical  experience  has  shown,  however,  that  in  general  it  is  not 
always  possible  to 
interchange  trans- 
formers of  the  two 
types  in  any  one 
set,  particularly  in 
quenched  spark  sets, 


FIG.  18. 


-M- 


Secondary 


FIG.  19. 


where  the  alterna- 
tor, transformer,  and 
condenser  of  the 
closed  oscillating 
circuit,  as  shown  in 
figure  74,  must  be  designed  as  a  whole  to  secure  the  best  results. 

Transformers  may  be  divided  into  two  types,  depending  on  the 
nature  of  the  insulation,  whether  oil  insulated  or  dry  insulated.  In 
the  first  the  transformer  is  completely  immersed  hi  a  suitable  in- 
sulating oil,  such  as  transil  oil,  in  an  iron  tank  provided  with  a 
cover  to  keep  the  oil  from  spilling,  through  which  the  terminals 


EADIOTELEGEAPHY. 


23 


extend,  strongly  insulated,  as  with  porcelain  for  example.  In  the 
second  type  strong  insulating  fabrics  or  materials  are  used  around 
and  between  the  windings  which  are  saturated  with  a  nonfluid  insu- 
lating compound. 

The  connections  of  the  transformer,  etc.,  are  shown  in  figure  20 
where  A  is  the  alternating  current  generator,  K  the  telegraph  key, 
T  the  transformer  with  primary  and  secondary  windings,  C  the  con- 
denser, S  the  spark  gap,  and  L  the  inductance. 

ALTERNATORS. 

The  transformer  receives  its  power  from  an  alternating  current 
generator,  or  alternator,  as  it  is  often  called,  which  is  either  belt 
driven  from  an  engine  or  electric  motor,  or  directly  driven  by  elec- 


-u 


+  G 


I 


FIG.  20. 

trie  motor,  in  which  case  the  two  machines  are  mounted  on  the  same 
bedplate  and^the  shafts  connected  by  a  flexible  coupling,  the  set  being 
called  a  motor-generator  set.  The  two  essential  parts  of  an  alternator 
from  an  electrical  point  of  view  are  the  fields  and  the  armature.  A 
direct  current  is  supplied  to  the  former  and  an  alternating  current 
is  delivered  by  the  latter.  Alternators  are  built  of  three  general 
types,  with  revolving  field,  revolving  armature,  and  of  the  inductor 
types,  of  which  the  last  two  are  generally  used  in  radio  work.  In  the 
revolving  armature  type  the  fields  are  stationary  and  the  armature 
rotates,  its  wires  thus  cutting  the  magnetic  lines  from  the  field  wind- 
ings and  genera  ting  the  alternating  current  which  is  brought  out  by 
brushes  bearing  on  two  collector  rings,  or  slip  rings,  as  they  are  called. 
In  the  inductor  type  both  the  field  and  the  armature  axe  stationary, 


24  RADIOTELEGRAPHY. 

the  rotating  part  being  simply  an  iron  form  with  projecting  pole 
pieces,  the  rotation  of  which  carries  the  magnetic  lines  from  the 
fields  in  and  out  of  the  fixed  armature,  the  wires  of  which  thus  cut 
the  magnetic  lines  and  generate  the  alternating  current.  In  this 
type  of  machine  there  are  no  revolving  wires  or  moving  contacts  of 
any  kind.  The  moving  part,  as  armature,  field,  or  inductor,  as  the 
case  may  be,  is  called  the  rotor.  The  stationary  part  is  called  the 
stator. 

The  alternator  fields  require  a  direct  current  for  their  energizing, 
which  may  be  furnished  either  by  an  outside  direct-current  source, 
such  as  the  direct-current  mains  that  supply  the  power  to  run  the 
direct-current  motor  of  a  motor-generator  set,  as  shown  in  figure  74, 
or  by  an  exciter,  which  is  a  small  direct-current  machine  that  may  be 
mounted  on  the  alternator  shaft  or  may  be  a  separate  machine  inde- 
pendently driven  by  any  convenient  means  as  shown  in  figures  78 
and  80. 

RHEOSTAT  AND  REACTANCE  CONTROL. 

In  order  to  control  the  power  delivered  to  the  transformer  a  vari- 
able resistance  or  rheostat  is  often  inserted  in  series  in  the  circuit  of 
the  alternator  armature  and  transformer  primary;  in  other  cases  a 
variable  inductance  called  a  reactance  or  reactance  regulator  is  used, 
consisting  of  coils  of  heavy  wire,  with  taps  brought  out  at  different 
points,  wound  on  a  laminated  iron  core.  The  rheostat  and  the  re- 
actance may  serve  similar  but  not  necessarily  the  same  purpose ;  thus 
increasing  the  resistance  in  the  rheostat  always  decreases  the  power 
delivered  to  the  transformer,  and  increasing  the  reactance  may  do 
likewise.  In  these  cases  the  rheostat  or  reactance  may  normally  be 
cut  out  of  circuit  and  introduced  only  as  needed  to  cut  down  the 
power,  as  for  example,  when  it  is  desired  to  decrease  the  range  of  a 
set  so  as  not  to  cause  interference  at  a  distant  station  or  when,  as 
required  by  law,  a  ship  station  reduces  its  power  as  it  comes  within 
15  miles  of  a  naval  or  military  station.  However,  increasing  the 
reactance  does  not  always  cut  down  the  power;  in  fact,  in  some  cir- 
cuits of  the  quenched-spark  type  it  may  actually  increase  the  power 
delivered  to  the  transformer,  and  hence  to  the  antenna,  where  it 
causes  an  increase  in  the  antenna  current. 

The  reason  for  this  is  that  there  is  a  combined  adjustment  of  the 
inductances  in  the  transformer  primary  and  secondary  circuits  and  of 
the  capacity  of  the  closed  circuit  condenser  which  is  best  adapted  for 
the  charging  of  this  condenser  at  regular  intervals.  In  some  cases 
more  inductance  is  required  than  that  in  the  alternator  armature,  and 
the  transformer  primary,  and  it  is  then  added  as  a  reactance  in  the 
primary  circuit.  In  other  cases  the  inductance  may  be  added  as  a  re- 
actance in  the  secondary  circuit,  where  evidently  the  coil  must  be  de- 


RADIOTELEGEAPHY.  25 

signed  to  withstand  high  potentials.  In  a  few  cases  reactances  are 
added  in  both  circuits  so  as  to  secure  the  desired  results.  When 
the  best  adjustments  have  been  attained  it  is  often  found  that  the 
transformer  primary  current  drops  to  a  minimum  value,  the  antenna 
current  rises  to  a  maximum,  and  at  the  same  time  the  note  of  the 
spark  is  the  clearest. 

KEYS. 

In  the  smaller  sizes  of  radio  sets  the  current  from  the  alternator 
to  the  transformer  can  be  controlled  by  ordinary  types  of  Morse  keys, 
with  either  silver  or  platinum  contacts,  without  troublesome  sticking 
or  arcing  even  at  fast  sending.  In  the  larger  sizes,  however,  special 
means  of  cutting  down  the  arc  at  the  breaking  of  the  circuit  must  be 
used,  such  as  shunting  the  key  by  a  resistance,  condenser,  reactance, 
etc.,  so  that  the  key  does  not  break  the  whole  current,  as  shown  in 
figure  74.  In  this  case,  however,  it  must  be  remembered  that,  as 
these  shunts  always  allow  some  current  to  flow  through  them,  the 
high-tension  and  high-frequency  circuits  are  alive  and  it  may  be 
dangerous  to  touch  any  of  them.  In  the  largest  sets  a  relay  Jcey  is 
generally  furnished,  which  consists  of  an  electromagnet  the  windings 
of  which  are  in  series  with  an  ordinary  Morse  key  and  a  source  of 
direct  current,  and  the  armature  of  which  carries  the  heavy  contacts 
necessary  to  break  the  current  in  use.  Such  a  key  may  be  used  to 
break  a  current  of  50  or  60  amperes  or  more  without  injurious  spark- 
ing. In  some  cases  a  single  large  key  with  contacts  an  inch  or  so  in 
diameter  and  a  handle  a  foot  long  has  been  used. 

Another  type  of  key  is  coming  into  use,  known  as  a  u  break  hey," 
which  permits  the  receiving  operator  to  break  the  transmitting 
operator  as  on  a  wire  line.  Among  other  ways  this  may  be  accom- 
plished by  providing  the  ordinary  key  with  an  extra  set  of  contacts 
which,  just  after  the  current  has  been  broken  in  making  a  dot  or 
dash,  and  just  as  the  key  handle  conies  up  to  its  final  position,  auto- 
matically connects  the  receiving  circuit  to  the  antenna  and  ground 
without  the  necessity  of  throwing  a  special  switch.  At  any  time  that 
the  receiving  operator  misses  a  word  or  desires  to  l  i  break ' '  the  trans- 
mitting operator  he  holds  his  key  down  or  calls  "bk,"  and  the  trans- 
mitting operator  with  the  telephones  on  his  head  and  with  his  detector 
in  adjustment  will  hear  the  call  between  the  dots  and  dashes  of  his 
own  sending  and  thus  be  broken.  For  most  successful  use  both 
operators  should  be  provided  with  break  keys.  It  is  essential  that 
the  receiving  circuits  in  general  and  the  detector  in  particular  be 
protected  from  sparks  from  the  transmitting  circuits,  and  that  the 
operators  be  not  bothered  by  the  sounds  from  their  spark  gaps  or 
machinery. 


26 


EADIOTELEGKAPHY. 


DEFINITIONS   OF   ALTERNATING-CURRENT  TERMS. 

For  a  proper  understanding  of  some  of  the  points  on  the  following 
pages  definitions  and  explanations  will  be  given  of  the  more  common 
terms  in  use  in  the  practice  of  alternating  currents. 

The  frequency  with  which  the  charges  in  the  condenser  C  of 
figure  20  are  renewed  by  the  transformer  depends,  among  other 
things,  upon  the  rate  at  which  the  voltage  and  current  delivered  by 
the  alternator  is  varying.  Figure  21  represents  the  manner  in  which 
these  quantities  vary,  where  the  set  of  values  ABODE,  half  of  which 
is  positive  and  half  negative,  is  called  a  cycle  of  voltage  or  current, 
the  symbol  for  which  is  often  thus  written  ~ .  The  number  of  cycles 
per  second  is  called  the  frequency  and  the  letter  "n"  is  often  used 
as  its  symbol.  In  commercial  alternators  used  in  radio  telegraphy 
the  frequencies  are  generally  60,  120,  480,  or  500  cycles  per  second; 
that  is,  there  are  60,  120,  etc.,  complete  sets  of  values,  such  as  ABODE 


:B 


\ 


Time 


\ 


Fia.  21. 


of  figure  21  per  second,  or  n  =  60,  120,  etc.  Half  a  cycle,  such  as  the 
set  of  values  ABC  or  ODE  of  figure  21,  which  may  be  either  posi- 
tive or  negative,  is  called  an  alternation.  There  are  always  twice 
as  many  alternations  per  second  as  there  are  cycles.  The  frequency 
of  an  alternating  current  is  sometimes  given  in  alternations  per 
minute  instead  of  cycles  per  second,  thus  a  current  of  60  cycles  per 
second  is  of  the  same  frequency  as  one  of  7,200  alternations  per 
minute.  The  time  taken  to  complete  one  cycle  is  called  the  period, 
and  the  letter  T  is  often  used  as  its  symbol,  thus  if  there  are  500 
cycles  per  second,  the  time  to  complete  one  cycle  is  1/500  second  or 
0.002  second;  that  is,  T^^  second  orT=  0.002  second.  Similarly 
the  time  for  one  alternation  of  a  current  of  the  same  frequency 
is  1/1,000  second  or  0.001  second.  The  relation  between  the  fre- 
quency in  cycles  per  second  and  the  period  in  fractions  of  a  second 


is  given  by  the  formulae  T 


or  N  =    . 


RADIOTELEGRAPHY.  27 

The  highest  value  of  the  current  or  voltage  in  any  alternation, 
as  at  points  B,  D,  etc.,  of  figure  21  or  the  corresponding  points  in 
figures  13,  14,  and  15,  is  called  the  amplitude  or  sometimes  the  peak 
of  the  curve. 

The  frequency  of  the  alternating  current  is  sometimes  indicated 
by  a  frequency  meter,  which  in  one  type  consists  of  a  series  of  flat 
steel  springs  or  reeds,  each  with  a  different  period  of  mechanical 
vibration  which  is  marked  on  it,  the  whole  series  covering  a  range  of 
frequency  of  from,  say,  470  to  530  vibrations  per  second.  Behind  the 
springs  is  an  electromagnet  carrying  the  alternating  current,  the  fre- 
quency of  which  is  to  be  measured.  When  the  frequency  of  the 
electromagnetic  impulses  is  the  same  as  that  of  any  one  of  the  reeds 
it  is  set  into  vibration  by  resonance  with  these  impulses,  and  the 
frequency  of  the  current  is  then  the  same  as  that  marked  on  the  reed 
in  vibration. 

It  will  be  noted  that  there  is  a  similarity  between  the  sustained 
oscillations  as  represented  in  figure  15  and  the  alternating  current 
or  voltage  as  represented  in  figure  21.  The  two  curves  have  the  same 
shape  or  form,  being  known  in  trigonometry  as  sine  curveSj  but  they 
differ  in  the  greatly  increased  frequency  of  a  hundred  thousand  or 
million  per  second  in  the  radio  circuits  (the  closed  and  open  oscil- 
lating circuits),  as  compared  with  that  of  60  to  500  per  second  in 
the  power  circuits  (the  alternator  and  transformer  circuits).  It  is 
the  general  practice  to  speak  of  the  number  of  oscillations  or  of  cycles 
per  second  in  radio  circuits,  but  only  of  the  number  of  cycles  per 
second  in  power  circuits. 

HIGH-FREQUENCY  CIRCUITS. 
CLOSED   OSCILLATING   OR   PRIMARY   CIRCUIT. 

The  circuit  of  coil  L,  condenser  C,  and  spark  gap  S,  as  shown  in 
heavy  lines  in  figure  20,  is  called  the  closed  oscillating  or  primary 
circuit,  as  distinguished  from  the  open,  radiating,  or  secondary  cir- 
cuit to  be  described  later.  These  three  elements  are  always  connected 
in  series  to  form  the  circuit,  which  is  found  in  all  spark  excitation 
types  of  radio  stations.  There  are  two  different  methods  of  con- 
necting the  transformer  secondary  leads  to  this  circuit  for  the  charg- 
ing of  the  condenser,  one  of  which  is  shown  in  the  upper  part  of 
figure  20  where  the  condenser  is  seen  to  be  directly  across  the  trans- 
former secondary  leads,  and  the  other  in  the  lower  part  where  the 
spark  gap  is  so  connected.  In  this  latter  case  the  condenser  is 
charged  through  the  inductance  L,  but  its  resistance  and  inductance 
are  so  small  as  compared  with  that  of  the  transformer  secondary  as 
to  have  no  effect  in  the  charging.  There  is  no  essential  difference  in 
the  operation  of  the  two  types  of  connections. 


28 


RADIOTELEGRAPHY. 


The  actions  taking  place  in  the  closed  circuit  as  a  whole  are  as  fol- 
lows: The  condenser  begins  to  get  its  charge  at  the  beginning  of 
each  alternation,  as  at  points  A,  C,  E,  etc.,  of  figure  21,  and  reaches 
such  a  potential  as  to  cause  its  discharge  across  the  gap  and  through 
the  inductance  at  the  peaks  of  the  curve,  as  at  points  B,  D,  etc.  The 
condenser  is,  so  to  speak,  a  reservoir  which  is  filled  and  discharged 
1,000  times  per  second  in  a  500-cycle  alternator  set.  In  figure  22  the 
upper  curve  represents  the  500-cycle  alternating  current  delivered  by 
the  transformer  secondary  to  the  condenser  which  is  charged  thereby; 
the  lower  curve  represents  the  discharge  of  the  condenser,  produc- 
ing damped  wave  trains  of  perhaps  20  or  30  oscillations,  each  train 
lasting  a  few  millionths  or  hundred  thousandths  of  a  second,  as 
shown  in  figures  13  and  14.  In  order  to  be  able  to  show  the  wave 
trains  at  all  in  figure  22  their  duration  must  be  shown  much  exag- 


-3KT  Sec- 


FIG.  22. 

gerated  as  compared  with  the  intervals  between  them.  Thus,  if  the 
period  of  each  complete  oscillation  in  the  train  were  -g00?ooo  second 
and  there  were  25  oscillations  in  the  train,  each  train  would  persist 
f°r  5 o o2, o o o  second,  or  2oooo  second,  or  the  duration  of  each  wave  train 
is  only  one-twentieth  of  that  between  successive  trains. 

It  must  be  rioted  that  although  the  transformer  secondary  is  con- 
nected to  the  closed  oscillating  circuit,  as  shown  in  figure  20,  it  takes 
no  part  in  the  oscillations  of  this  circuit.  The  reason  for  this  is  that 
the  period  of  the  circuit  of  transformer  secondary  and  closed  circuit 
capacity  is  so  long  (in  fractions  of  a  second)  on  account  of  the  large 
secondary  inductance  that  the  wave  train  in  the  closed  oscillating 
circuit  has  been  completed  before  the  transformer  secondary  circuit 
has  had  time  to  complete  a  part  of  one  of  its  own  slow  oscillations. 


EADIOTELEGEAPHY.  29 

The  period  or  frequency  of  the  oscillations  of  the  closed  circuit  is 
thus  independent  of  the  transformer  circuit. 

In  the  preceding  example  it  has  been  assumed  that  there  was  one 
discharge  in  each  alternation  or  two  discharges  per  cycle;  that  is, 
1,000  wave  trains  per  second.  In  some  cases,  however,  the  circuit 
may  be  arranged  so  that  there  is  a  charge  and  discharge  in  every 
other  alternation — that  is,  only  one  discharge  per  cycle — which,  with 
a  500-cycle  alternator,  would  give  only  500  wave  trains  per  second. 
In  both  cases,  however,  the  wave  trains  are  separated  by  equal  inter- 
vals of  time.  When  the  wave  trains  are  thus  separated  by  equal 
intervals  of  time  the  note  of  the  spark  is  said  to  be  pure.  In  some 
cases,  however,  it  is  possible  to  charge  the  condenser  two,  three,  or 
even  more  times  per  alternation,  and  hence  four,  six,  or  even  more 
times  per  cycle,  and  then  it  is  said  that  these  are  multiple  discharges. 
Under  these  circumstances  the  intervals  of  time  between  the  wave 
trains  will  not  in  general  be  all  equal  and  the  note  will  not  be  pure. 
The  pure  note  is  often  very  desirable,  although  not  always  neces- 
sary in  practical  work. 

WAVE     TRAIN     OR     SPARK     FREQUENCY. 

The  number  of  wave  trains  per  second  is  called  the  wave-train 
frequency  or  the  spark  frequency.  If  the  alternator  frequency  is 
500  cycles  per  second  and  there  is  a  discharge  once  in  every  alterna- 
tion, or  1,000  discharges  per  second,  the  spark  frequency  is  1,000  per 
second.  It  must  be  noted  that  in  general  the  alternator  frequency 
and  the  wave-train  frequency  are  not  the  same;  in  fact,  they  may  be 
very  different,  as  in  the  case  of  multiple  discharges  mentioned  in  the 
last  paragraph. 

If  the  spark  frequency  is,  say,  120  per  second,  as  from  a  60-cycle 
alternator,  it  is  said  to  be  low,  but  if  it  is  1,000  per  second,  as  from 
a  500-cycle  alternator,  it  is  said  to  be  high.  There  are  certain  advan- 
tages in  a  high  spark  frequency  which  appear  both  at  the  trans- 
mitting and  at  the  receiving  stations.  If  the  closed  circuit  con- 
denser is  charged  1,000  times  per  second  to  a  certain  potential,  it  is 
evident  that  more  energy  will  be  required  than  if  charged  only  120 
times,  the  formula  for  the  energy  being  1/2  C  V2N,  where  C  is  the 
capacity,  V  the  potential,  and  N  the  number  of  times  per  second. 
If  the  same  amount  of  energy  is  available  in  the  two  cases — that  is, 
if  1/2  C  V2N  is  constant — the  smaller  the  value  of  N  the  larger 
must  be  the  value  of  V,  other  conditions  being  constant,  and,  vice 
versa,  the  larger  the  value  of  N  the  smaller  may  be  the  value  of  V. 
The  earlier  practice  was  to  make  N  small,  as  120  per  second  from  a 
60-cycle  alternator,  and  V  large,  as  30,000  volts.  The  modern  prac- 
tice is  to  make  N  large,  as  1,000  from  a  500-cycle  alternator,  and  V 


30  RADIOTELEGRAPHY. 

small,  which  in  this  example  must  be  about  10,800  volts.  It  is  evi- 
dent, then,  that  the  transformer  secondary  and  the  closed  oscillating 
circuit  condenser  do  not  need  to  be  built  to  withstand  the  high 
voltages  formerly  used,  and  that,  therefore,  they  may.  be  lighter  and 
more  compact;  also  that  the  oscillation  transformer  and  antenna,  to 
be  described  later,  do  not  need  the  very  high  insulation  which  was 
formerly  necessary. 

The  advantages  of  the  high  spark  frequency  at  the  receiving 
station  will  be  mentioned  later  under  that  heading. 

TRANSMITTING   CONDENSERS. 

A  brief  description  of  the  three  elements,  condenser,  coil,  and 
spark  gap,  will  be  giyen. 

The  functions  of  the  condenser  are,  by  virtue  of  its  capacity,  to 
store  the  charge  delivered  to  it  by  the  transformer  secondary  circuit 
until  its  potential  reaches  the  desired  value  as  determined  by  the 
spark  gap,  and  then  to  discharge  through  the  gap  and  the  inductance. 
An  ideal  condenser  would  be  one  that  was  perfectly  insulating,  could 
not  be  punctured,  and  showed  no  heating  or  losses  of  any  kind  during 
charging  and  oscillatory  discharging. 

There  are  several  different  types  of  transmitting  condensers  used 
in  the  Signal  Corps  radio  stations,  varying  widely  in  capacity,  size, 
voltage,  etc.,  from  the  small  mica  ones  of  the  field  radio  sets  to  the 
compressed-air  type  in  the  permanent  stations.  All  types  consist 
essentially  of  two  conducting  surfaces  separated  by  an  insulator  or 
dielectric,  as  it  is  often  called,  which  can  withstand  the  high  voltage 
required  to  break  down  the  spark  gap  without  puncturing.  Prob- 
ably the  most  efficient  condenser  is  the  compressed-air  type,  which 
consists  of  a  large  number  of  circular  metal  plates  mounted  on  two 
sets  of  supports  with  a  small  air  space  between  each  plate,  the  top 
plate  and  every  alternate  plate  being  connected  together  as  one  set 
and  the  remaining  plates  as  the  other  set.  The  whole  is  contained 
in  an  air-tight  tank,  one  set  of  plates  being  connected  to  the  tank 
as  one  terminal  and  the  other  set  to  a  terminal  brought  out  through 
the  cover  in  a  sealed  porcelain  insulator.  Air  is  then  pumped  into  the 
tank  until  a  pressure  of  about  240  pounds  per  square  inch  is  reached, 
or  about  16  atmospheres  of  15  pounds  per  square  inch,  as  shown  by 
a  pressure  gauge  on  top  of  the  tank.  At  this  pressure  it  has  been 
found  that  air  has  an  insulating  strength  many  times  greater  than 
at  ordinary  pressures.  Condensers  of  this  type  will  withstand  a 
maximum  or  "peak"  voltage  of  about  20,000  volts  under  service 
conditions.  The  most  serious  objection  is  the  excessive  weight,  a  tank 
of  about  0.006-microfarad  capacity  weighing  about  300  pounds. 

There  are  many  types  of  condensers  using  glass  as  the  dielectric, 
such  as  plates  or  jars  covered  with  foil  or  plated  with  copper.  When 


KADIOTELEGRAPHY.  31 

these  condensers  are  used  at  high  potential,  such  as  25,000  volts  or 
more,  there  is  developed  at  the  sharp  edges  of  the  foil  or  plating  a 
discharge  (sometimes  called  brush  discharge),  which  spreads  out  over 
the  surface  of  the  glass  and  is  accompanied  by  a  hissing  sound  and 
considerable  heating  of  the  glass  close  to  the  edges.  In  a  dark  room 
it  will  be  seen  that  the  edges  are  illuminated  by  a  pink  light.  The 
puncturing  of  the  glass  and  the  breaking  down  of  the  condenser  often 
takes  place  close  to  the  edges,  due  probably  to  the  brush  discharge  and 
the  local  heating  of  the  glass.  These  discharges  represent  losses 
which,  in  part  at  least,  can  be  prevented  by  covering  the  edges  of  the 
foil  with  an  insulating  coating,  such  as  asphaltum,  and  more  com- 
pletely by  immersing  the  condensers  in  an  insulating  oil,  such  as 
castor  oil,  etc. 

The  capacity  of  these  condensers  and  the  voltage  which  they  can 
withstand  depend  so  much  on  the  quality  of  glass,  the  manner  in 


I 

1 


FIG.  23. 


which  it  was  annealed,  its  thickness,  etc.,  that  it  is  impracticable  to 
give  figures  except  for  condensers  that  have  actually  been  tested.  In 
the  case  of  a  good  grade  of  plate  glass  about  -3^-  inch  thick,  free  from 
scratches,  bubbles,  etc.,  a  potential  of  20,000  to  25,000  volts  can  be 
safely  used.  In  figure  23  is  shown  a  closed  oscillating  circuit  with 
three  condenser  jars  connected  in  parallel;  that  is,  the  three  outside 
coatings  are  connected  together  as  one  terminal  and  the  three  inside 
coatings  as  the  other,  and  with  a  potential  of  20,000  volts  between  the 


T 


|*l  5000  V+l  5000  V.»| 
\* — 30000  Volts -^ 


FIG.  24. 


terminals.  If  the  condensers  break  down  at  this  potential  or  if 
higher  potentials,  such  as  30,000  volts,  are  to  be  used,  they  should  be 
connected  in  series-parallel  as  shown  in  figure  24,  where  two  banks, 


32 


EADIOTELEGRAPHY. 


each  of  three  jars  in  parallel,  are  connected  in  series.  It  is  to  be  noted 
that  this  connection  requires  twice  as  many  jars  as  before,  but  if  the 
total  potential  is  30,000  volts,  the  potential  across  each  jar  is  now 
only  15,000  volts  instead  of  20,000  as  before.  This  connection,  how- 
ever, reduces  the  capacity  to  one-half  of  its  former  value,  so  that  to 
retain  the  same  capacity  as  before  two  banks  each  of  six  jars  must  be 
connected  in  series-parallel,  as  shown  in  figure  25,  thus  requiring  four 
times  as  many  jars  as  the  first  circuit. 

The  capacity  of  one  glass  plate  about  -f$  inch  thick  and  with  the 
foil  15  inches  square  is  about  0.0020  to  0.0025  microfarad.  The  capac- 
ity of  a  jar  with  glass  J  inch  thick,  4f  inches  in  diameter,  and  height 
of  foil  of  10  inches  is  about  0.002  M.  F. 

Another  type  of  condenser  having  some  advantages  is  the  Moscicki 
jar,  which  consists  essentially  of  a  glass  tube  or  jar  with  inside  and 
outside  coatings,  as  in  the  other  types,  but  at  the  edges  of  the  coatings 
where  the  puncture  usually  takes  place  the  glass  is  thickened  to  give 
increased  strength,  and  at  the  same  time  the  edges  are  covered  with 


I 
1 


k|  5000  v*l  5000  V*l 
k- 30  000  Volts  H 


FIG.  25. 

an  insulating  liquid  to  stop  the  brush  discharge.  The  whole  is  con- 
tained in  a  brass  tube  to  which  the  outside  coating  is  connected,  the 
inside  coating  being  brought  out  to  a  binding  post  through  a  sealed 
porcelain  insulator.  The  case  and  the  binding  post  thus  become  the 
two  terminals.  These  tubes  are  made  in  two  sizes,  the  larger  of  which 
is  in  more  general  use,  has  capacity  of  about  0.005  M.  F.,  and  is  capa- 
ble of  withstanding  20,000  volts. 

There  are  many  other  types  of  condensers  using  such  dielectrics  as 
mica,  paper,  and  various  molded  insulating  compounds.  In  a  few 
cases  oil  is  used  as  the  dielectric,  in  which  case  metal  plates  are 
mounted  on  insulating  supports  a  short  distance  apart  in  tanks  filled 
with  a  suitable  insulating  oil,  such  as  castor  oil,  etc. 

TRANSMITTING   INDUCTANCES. 

The  function  of  the  inductance  is  to  form  one  of  the  two  elements, 
the  condenser  being  the  other,  necessary  for  developing  and  main- 
taining the  oscillations,  and  to  serve  as  a  means  of  transferring  energy 


EADIOTELEGKAPHY. 


33 


from  one  circuit  to  another.  An  ideal  coil  would  be  one  having  the 
desired  inductance  but  with  a  zero  resistance  to  the  oscillating 
currents. 

The  inductance  coil  L,  which  has  been  shown  in  the  various  figures, 
may  be  any  one  of  several  different  types,  such  as  a  helix  of  heavy 
copper  wire,  thin-walled  copper  tubing,  or  flat  strips,  or  &flat  spiral 
of  copper  ribbon,  such  as  the  linking  coil  of  the  Signal  Corps  field 
radio  sets,  etc.  These  are  generally  provided  with  clips  so  as  to  be 
able  to  vary  continuously  the  number  of  turns,  and  hence  the  induc- 


.150 


.100 


.050 


10 


15 


Number  of  turns 


FIG.  2G. 


tanco  in  circuit.  In  some  cases,  however,  the  coil  may  be  provided 
with  plugs  and  sockets  to  vary  the  inductance  by  steps  and  other 
means  provided  elsewhere  in  the  circuit  to  get  all  adjustments  be- 
tween the  steps. 

Curves  showing  how  the  inductance  of  a  coil  varies  with  the 
numbers  of  the  turns  in  circuit  is  called  a  calibration  curve  of  the 
inductance.  In  figure  26  is  shown  such  a  curve  for  a  helix,  with 
square  turns  wound  with  copper  tubing  about  one-fourth  inch  in  di- 
ameter, the  length  of  each  side  being  2H  inches  and  the  spacing  of  the 
turns  being  1  inch  between  centers.  In  figure  27,  A  and  B,.are  shown 
92448—15 3 


34 


RADIOTELEGRAPHY. 


two  calibration  curves  of  a  flat  spiral,  similar  to  the  one  used  in  the 
field  radio  sets,  in  the  first  of  which  (A)  the  turns  are  counted  from 
the  outside  inward,  and  in  the  second  (B)  they  are  counted  from  the 
inside  outward.  Thus  it  is  seen  that  in  using  different  numbers  of 
turns  in  a  flat  spiral  care  must  be  taken  to  state  how  the  turns  are 
counted. 

There  is  another  useful  type  of  inductance  called  the  variometer, 
which  consists  essentially  of  two  coils  connected  in  series  or  parallel, 
as  desired,  one  of  which  is  movable  with  respect  to  the  other.  In 
some  cases  one  coil  is  arranged  to  slide  past  the  other  in  a  plane 
parallel  to  its  windings,  as  indicated  in  figure  28;  in  other  cases  one 


.120 
.110 
.100 
.090 
,0.080 
^.070 
^.060 
^.050 
'^  -040 
|.030 
|.020 

-^  .010 

0 

^1 

x 

' 

1 

/ 

1 

1 

s 

/ 

/ 

/ 

i 

y 

/ 

/ 

J 

/ 

/ 

/A 

/ 

/ 

, 

/B 

/ 

/ 

/ 

/ 

x 

/ 

^ 

•x 

-^ 

X 

=r 

.  —  - 

^~ 

^^ 

o      a 

0      2 


468 

Number  of  turns 


FIG.  27. 


coil  is  rotated  inside  the  windings  of  the  other,  as  indicated  in  figure 
29.  In  the  second  type,  when  the  coils  are  in  the  same  plane  and  the 
windings  are  connected  so  that  the  current  is  circulating  through 
them  in  the  same  direction,  the  two  magnetic  fields  are  helping  each 
other  and  the  inductance  is  a  maximum;  if,  now,  one  coil  is  rotated 
through  an  angle  of  180  degrees  the  two  fields  are  opposing  and  the 
inductance  is  a  minimum;  for  intermediate  angles  the  inductance 
will  have  some  intermediate  value.  The  variometer  thus  has  the 
advantage  of  giving  a  continuous  change  of  inductance  without 
moving  clips  or  contacts,  but  has  what  may  ~be  under  certain  condi- 
tions the  disadvantages  of  not  giving  zero  inductance  at  its  minimum 


KADIOTELEGKAPHY. 


35 


position  and  of  always  having  the  resistance  of  all  its  wire  in'circuit. 
A  variometer  is  generally  used  in  connection  with  a  helix  or  coil, 


FIG.  28. 


variable  only  by  steps,  to  give  intermediate  values  of  the  inductance 
as  mentioned  above,  and  shown  in  figure  77. 

The  earlier  types  of  closed  circuit  inductance  were  wound  with 
wire  or  tubing,  the  resistance  of  which  to  direct  current  was  very 


FIG.  29. 


low.     Both  theory  and  experiment  have  shown,  however,  that  the 
resistance  to  high-frequency  currents  may  be  comparatively  large. 


36 


RADIOTELEGRAPH  Y. 


The  explanation  is  that  these  high-frequency  currents  tend  to  travel 
almost  wholly  on  the  surface  of  the  conductor  and  do  not  penetrate 
to  any  considerable  distance  into  the  wire.  Thus  a  thin-walled  tube 
will  have  practically  the  same  resistance  to  high-frequency  currents 
as  a  solid  wire  of  the  same  diameter,  the  inside  of  the  wire  carrying 
no  current  at  all. 

This  tendency  of  the  current  to  flow  only  on  the  outer  surface  is 
sometimes  called  the  liskin  effect"  and  the  distance  to  which  the 
current  penetrates  the  thickness  of  the  skin.  The  higher  the  fre- 
quency the  more  marked  is  the  skin  effect  and  the  thinner  is  the 
skin;  in  other  words,  the  higher  the  frequency  the  larger  will  be  the 

A/o.  O  Wire.  B.  &  S.  Gauge 


40 

en  _. 

Number  of  Times  of  Increase  in  Resistance 

—  _'  f\>  !\>  U)  Ui 

puiouiowOtf 

^_ 

^ 

^ 

^ 

^ 

/ 

^ 

A 

/ 

/ 

0 

\ 

>             3                             5             6             7             8             9            K 

Frequency  Jn  Hundred  thousand  cycles 

FIG.  30. 

resistance  for  the  same  size  and  length  of  wire.  In  figure  30  is  given 
the  curve  showing  the  increase  in  resistance  for  No.  0  copper  wire, 
B.  &  S.  gauge  (about  325  mils  in  diameter),  as  the  frequency  changes 
from  zero  or  a  steady  current  up  to  1,000,000  cycles  per  second. 
Thus  at  500,000  cycles  it  is  seen  that  the  resistance  has  been  increased 
about  22  times  the  D.  C.  value.  The  scale  of  such  a  curve  will  differ 
with  the  different  sizes  of  wire,  the  increase  being  greater  than  here 
shown  for  wires  larger  than  No.  0  and  less  for  smaller  sizes.  In  fig- 
ure 31  is  given  the  curve  showing  the  increase  in  resistance  for  the 
various  sizes  of  copper  wire  in  the  B.  &  S.  gauge  at  a  frequency  of 
500,000  cycles  per  second.  Thus  a  wire  as  small  as  No.  35,  B.  &  S., 


RADIOTELEGRAPHY. 


37 


has  very  nearly  the  same  resistance  at  this  frequency  as  at  a  steady 
current,  or,  in  other  words,  -the  thickness  of  the  skin  at  this  fre- 
quency is  about  equal  to  the  radius  of  the  wire.  In  order  to  be  able 
to  include  all  sizes  of  wire  at  all  frequencies  it  is  evident  that  a  large 
number  of  curves  or  an  extensive  table  of  resistance  and  frequency 
would  be  necessary. 

If  a  large  number  of  wires,  the  diameter  of  which  is  such  that  the 
current  just  penetrates  to  the  center  at  any  given  frequency,  is  used 
in  parallel  in  the  form  of  a  compactly  stranded  wire  or  cable  it  is 
evident  that  all  the  copper  is  in  use  and  that  the  current-carrying 
surface  of  such  a  cable  is  very  much  greater  than  that  of  a  solid  wire 

Frequency  5OOOOO 


20 


\ 


0  5  10  15  20  25  3O  35  40 

<3/zes  of  wires:— B.  <£  5.  Gauqe 
FIG.  31. 

of  the  same  outside  diameter,  and  hence  the  resistance  is  very  much 
lower.  Each  wire  must,  however,  be  separately  insulated,  as  other- 
wise the  current  will  immediately  seek  the  outer  surfaces  of  the  outer 
wires  on  account  of  the  skin  effect,  and  the  resistance  will  not  be  much 
decreased  from  that  of  a  solid  wire.  Such  a  stranded  wire  or  cable, 
with  its  individual  wires  separately  insulated,  as  with  enamel,  is  some- 
times called  litzendraJit,  from  the  German  word.  The  number  of 
wires  depends  upon  the  current  to  be  carried  and  the  resistance  de- 
sired. In  the  smaller  sizes  it  is  generally  a  multiple  of  7,  as  7X7,  or 
49  wires,  and  in  the  larger  sizes  for  heavy  currents  the  number  may 
be  in  the  hundreds  or  even  thousands. 


38  RADIOTELEGRAPH  Y. 

It  is  evidently  impossible  to  get  a  continuously  variable  inductance 
by  a  sliding  clip  or  contact  on  all  the  wires  of  a  litzendraht  coil,  so 
that  when  such  an  inductance  of  low  resistance  is  desired  it  is  gen- 
erally made  in  the  form  of  a  variometer  wound  with  litzendraht. 
Many  modern  sets,  particularly  those  of  the  quenched-spark  type  of 
the  Telefunken  Co.,  use  such  coils. 

The  use  of  litzendraht  is  not  confined  to  transmitting  coils,  but 
is  also  used  in  receiving  sets  to  get  low-resistance  circuits. 

SPARK   GAPS. 

The  function  of  the  gap  is  to  serve  as  a  trigger  in  starting  the  oscil- 
lations and  to  limit  the  potential  applied  to  the  condensers  by  the 
transformer  secondary.  An  ideal  gap  would  be.  one  having  an  infinite 
resistance  during  the  charging  of  the  condensers  and  a  zero  resistance 
during  each  wave  train  of  the  discharge. 

The  types  of  spark  gaps  in  use  differ  nearly  as  much  as  the  other 
parts  of  the  closed-circuit  elements.  In  small-sized  sets  the  electrodes 
or  terminals  are  generally  made  of  zinc  or  brass,  the  sparking  sur- 
faces being  either  balls  of  one-half  inch  diameter  or  more,  or  else 
rounded  surfaces.  Sharp  points  are  not  used,  as  at  small  separations 
the  potential  required  to  break  down  the  gap  is  too  small  to  allow  any 
considerable  power  to  be  used,  and  if  the  gap  is  opened  to  increase 
the  potential  and  power  the  gap  resistance  becomes  too  high.  As  the 
power  delivered  to  the  transformer  is  increased  it  is  soon  found  that 
the  discharge  at  the  gap  becomes  flaming  in  character  and  has  a 
hissing  sound,  seeming  to  be  more  like  an  arc  than  a  spark,  and  the 
gap  terminals  become  very  hot.  The  reason  for  this  is,  that  owing 
to  the  great  quantity  of  electricity  discharged  across  the  gap  the 
resistance  becomes  so  low  that  a  high-potential  alternating-current 
arc,  which  is  almost  a  short  circuit,  is  maintained  at  the  transformer 
secondary  terminals.  This  arc  is  formed  in  the  heated  air  and  the 
vapor  of  the  metals  forming  the  gap  terminals.  Experiment  has 
shown  that  a  blast  of  air  across  or  through  the  gap  will  blow  out 
the  arc  but  not  the  spark.  By  thus  removing  the  short  circuit  the 
condenser  can  be  charged  to  the  full  potential  of  the  secondary  and  the 
power  of  the  set  increased — in  some  cases  it  may  be  nearly  doubled. 

The  air  blast  may  be  obtained  from  a  blower  or  compressor  driven, 
for  example,  by  an  electric  motor  or  directly  by  the  rotating  of  the 
gap  terminals  themselves,  in  which  case  it  is  known  as  a  rotating 
yap.  There  are  two  general  types  of  rotating  gaps,  in  the  first  of 
which  the  rotation  is  simply  a  convenient  means  of  giving  the  neces- 
sary ventilation  and  cooling.  It  is  not  necessary  that  it  be  provided 
with  rotating  terminals,  although  it  may  be  so  provided.  In  one  of 
the  early  types  used  in  the  Signal  Corps,  shown  in  figure  32,  a  rotating 
disk  is  used  between  two  fixed  terminals.  In  this  case  the  sparks  shift 


RADIOTELEGRAPH  Y.  39 

from  place  to  place  on  the  edges  of  the  disk  as  it  turns,  the  ven- 
tilation being  by  means  of  fans  on  the  face  of  the  disk,  which  blow 
the  air  away  from  the  gaps.  As  no  attempt  is  made  to  secure  any 
special  time  relation  between  the  discharges  and  the  alternator  fre- 
quency this  type  of  gap  is  often  called  a  nonsynchronous  gap. 

In  the  second  type  of  rotating  gap  one  set  of  electrodes  is  attached 
to  the  alternator  shaft,  preferably  insulated  from  it,  and  thus  rotates 
at  the  same  speed  as  the  armature;  the  other  terminal  is  mounted 
so  as  to  be  capable  of  adjustment,  both  in  the  direction  of  rotation 
and  in  a  radial  direction.  If  the  spacing  of  the  revolving  terminals 
is  such  that  there  are  as  many  terminals  pass  the  fixed  terminal  per 
second  as  there  are  alternations  per  second,  and,  further,  if  the  adjust- 
ments of  potential,  etc.,  are  such  that  the  discharge  is  at  the  peak  of 
each  alternation,  then  there  will  be  as  many  sparks  per  second  as 
there  are  alternations,  and  the  gap  is  called  a  synchronous  gap. 

In  order  to  secure  the  correct  adjustments  of  a  synchronous  gap 
the  fixed  terminal  should  be  adjusted  radially  to  give  only  a  small 


FIG.  32. 

clearance,  as  -^2  mcn  °r  less,  and  then  adjusted  in  the  direction 
of  rotation  as  follows:  If  the  rotating  terminals  are  watched  by  the 
light  of  the  sparks  themselves,  they  will  appear  either  to  be  waver- 
ing back  and  forth  or  else  to  be  nearly  fixed  in  position.  In  the 
former  case  the  discharge  does  not  occur  at  the  peak  of  the  wave,  but 
perhaps  before  the  peak  in  one  alternation  and  after  in  the  next, 
and  hence  the  wavering  appearance;  in  the  latter  case  the  discharge 
is  at  the  peak  of  the  wave  as  shown  by  the  apparent  steadiness  of 
position.  At  the  same  time  that  this  correct  adjustment  is  secured 
the  note  of  the  spark  as  heard  either  in  the  station  itself  or  at  a 
distant  receiving  station  will  become  much  clearer,  the  advantages  of 
which  will  be  mentioned  later. 

As  it  is  generally  best  not  to  have  long  leads  from  the  spark  gap 
to  the  other  elements  of  the  closed  circuit,  it  may  be  necessary  to 
have  all  of  the  closed  circuit  as  well  as  the  open  circuit  in  the  room 
with  the  alternator,  in  which  case  the  operator  and  the  receiving  set 
should  be  in  another  room.  In  some  cases  it  may  be  possible  to  mount 
the  alternator  and  gap  so  that  short  leads  can  be  brought  out  from 


40 


BADIOTELEGEAPHY. 


the  latter  through  well-insulated  bushings  into  the  next  room,  which 
should  be  sound  proof,  and  thus  all  the  circuits  be  contained  in  the 
same  room  with  the  operator  for  convenience  and  promptness  in 
making  changes  in  wave  length  and  other  adjustments,  etc. 


QUENCHED    SPARK    GAPS. 


Most  modern  sets  use  the  quenched  spark  gap,  a  brief  description 
of  which  will  be  given  here  and  the  theory  of  the  quenched  spark 
transmitter  later.  The  gap  is  essentially  a  series  gap  consisting  of  a 


FIG.  33. 


number  of  plates  with  small  separations  between  the  sparking  sur- 
faces, which  are  inclosed  in  air-tight  chambers  formed  between  the 
plates  themselves. 

In  figure  33  is  shown  a  section  of  a  gap  where  P  are  the  plates 
often  made  of  copper,  which,  on  account  of  good  conductivity  for 
heat,  will  carry  off  the  heat  of  the  spark;  F  are  the  flanges,  which 
help  the  cooling  by  exposing  a  large  area  to  the  air  or  to  the  air 
blast  to  be  mentioned  later;  S  are  the  sparking  surfaces  between 
which  the  sparks  pass,  which  may  be  of  the  same  copper  stock  as  the 
rest  of  the  plate  or  of  heavy  silver  plate  fastened  in  place  at  S;  M 


Compressible 
GasAef 


the  separators  or  insulating  rings,  also  caUed  gaskets,  between  the 
plates,  often  made  of  mica,  about  0.010  inch  thick  (10  mils),  the 
thickness  of  which  determines  the  distances  between  the  sparking 
surfaces.  In  some  cases  the  separators  are  made  of  rubber  or  other 
insulating  materials  which  are  somewhat  compressible,  and  then  the 
bearing  surfaces  are  often  corrugated,  as  shown  in  figure  34,  so  that 
the  material  may  be  pressed  down  into  the  annular  spaces.  What- 
ever the  type  of  separator,  the  gap  as  a  whole  must  be  put  under 
strong  mechanical  pressure  so  that  the  air  shall  be  excluded  from  the 
sparking  surfaces,  the  reason  for  which  seems  to  be  that  these  sur- 
faces are  roughened  with  free  exposure  to  air,  and  an  arc  is  formed 
at  some  point  which  behaves  as  a  short  circuit  between  the  plates  and 


RADIOTELEGRAPH  Y.  41 

lowers  the  efficiency  of  the  gap.  In  order  to  keep  the  gap  cool  the 
flanges  of  the  plates  are  generally  blackened,  as  a  black  body  will 
cool  more  quickly  than  a  polished  body,  other  things  being  equal. 
In  the  larger-sized  sets  it  is  necessary  to  cool  the  gap  by  means  of  a 
blower  driven  by  a  motor  similar  to  the  type  used  in  blowing  out  the 
arc  of  an  open  gap.  The  potential  between  each  plate  of  a  gap 
assembled  as  above  is  about  1,000  volts.  This  may  be  measured  by 
finding  the  potential  across  several  gaps  by  means  of  the  needle  gap 
and  the  values  in  Table  I  and  then  dividing  this  potential  by  the 
number  of  the  gaps. 

Under  service  conditions  a  quenched  gap  should  be  taken  apart 
only  when  it  is  absolutely  certain  that  trouble  in  the  radio  circuits 
has  been  located  in  the  gap  itself,  as  shown,  for  example,  by  one  or  two 
of  the  plates  becoming  much  hotter  than  the  others,  or,  by  an  actual 
puncture  of  a  gasket  or  separator.  The  reason  for  not  taking  the  gap 
apart  frequently  seems  to  be  that  after  a  certain  time,  depending  on  the 
amount  of  use,  the  oxygen  of  the  air  contained  between  the  plates 
becomes  inactive  and  there  is  no  tendency  of  the  sparks  to  roughen 
the  sparking  surfaces  and  form  local  arcs,  but  rather  that  these  sur- 
faces are  worn  smooth  and  kept  bright  by  the  sparking  action.  If, 
however,  the  gaps  are  continually  being  taken  apart  air  will  be 
admitted  each  time,  and  the  gap  may  not  give  the  results  that  other- 
wise would  be  attained.  There  are  cases  where  quenched  gaps  have 
been  used  handling  heavy  traffic  daily  for  six  months  or  more  with- 
out the  necessity  of  being  taken  apart  once  during  that  time,  and  in 
one  of  the  Signal  Corps  sets  such  a  gap  has  now  been  in  service  for 
nearly  three  years  without  having  a  plate  or  gasket  replaced  or  even 
the  gap  taken  apart.  If,  however,  it  becomes  necessary  to  clean  the 
plates,  they  should  be  laid  face  down  on  fine  emery  cloth  or  paper  on 
Sijiat  surface  and  the  roughness  carefully  smoothed  off.  When  mica 
is  used  as  a  separator  the  bearing  surface  is  generally  flush  with  the 
sparking  surface,  and  particular  care  must  be  taken  to  keep  the  two 
plane  and  parallel  as  shown  by  a  straightedge.  Any  irregularities 
on  the  bearing  surface  will  admit  air  and  injure  the  gap,  no  matter 
what  pressure  may  be  put  on  the  plates.  Almost  all  gaps  are  pro- 
vided with  more  plates  than  should  be  used  under  service  conditions, 
the  extra  gaps  being  short-circuited  by  clips  for  that  purpose,  so  that 
when  any  one  gap  becomes  bad  it  can  be  temporarily  cut  out  of  cir- 
cuit without  the  necessity  of  taking  the  whole  gap  apart. 

CONNECTION    OF    CLOSED    OSCILLATING    OR    PRIMARY    CIRCUIT 
WITH   ANTENNA   CIRCUIT. 

In  the  original  transmitting  arrangement  of  Marconi  the  spark  gap 
was  inserted  between  the  antenna  and  ground,  the  transfoimer  second- 
ary terminals  being  connected,  one  to  the  antenna  and  the  other  to 


42 


RADIOTELEGRAPHY. 


the  ground,  as  shown  in  figure  35.  This  circuit  is  often  known  as 
the  plain  Marconi  antenna  or  aerial.  As  the  antenna  has  both  induc- 
tance and  capacity  it  forms  in  this  case  the  oscillating  circuit,  taking 
the  place  of  the  circuit  CSL  of  figure  20.  The  values  of  the  induc- 
tance and  the  capacity  vary  with  the  size,  shape,  etc.,  of  the  antenna; 
thus  for  a  small  antenna,  as  on  an  artillery  tug  or  in  a  portable  field 
set,  the  capacity  maybe  between  0.0006  and  0.0009  mf.,  and  the  induc- 
tance between  20,000  and  30,000  cms.  or  0.02  and  0.03  millihenrys; 
and  for  a  "f"  or  inverted  "|_"  antenna  on  180-foot  masts,  the 
capacity  may  be  as  large  as  0.0015  or  0.0020  mf.,  and  the  inductance 
30,000  to  60,000  cms.  or  0.030  to  0.060  millihenrys.  It  is  to  be  noted 
that  this  capacity  is  about  the  same  as  that  of  one  jar  described  on 


0 

O 


FIG. 35. 

page  32.     Only  in  the  largest  stations  is  the  capacity  of  the  antenna 
as  large  as  0.01  mf. 

From  its  position  and  shape  the  antenna  circuit  is  often  called  the 
open  circuit,  as  distinguished  from  the  closed  oscillating  or  primary 
circuit.  It  is  a  good  radiator  of  the  electrical  energy  imparted  to  it 
by  the  transformer,  but  its  small  capacity  makes  it  impossible  to 
store  a  large  charge  in  it,  and  consequently  at  each  discharge  across 
the  gap  there  is  comparatively  little  energy  available  for  radiation 
For  this  and  other  reasons  to  be  mentioned  later  this  circuit  is  not 
now  used  in  practical  radio  telegraphy. 

COUPLING. 

By  means  of  the  arrangement  shown  in  figure  36  a  large  charge 
may  be  stored  in  the  condenser  C,  much  larger  than  that  which  can 
be  stored  in  the  antenna  of  figure  35,  and  the  discharge  of  this  con- 
denser through  the  gap  S  and  the  inductance  L  will  produce  powerful 
oscillations  in  the  closed  oscillating  or  primary  circuit.  On  account 


RADIOTELEGRAPH  Y. 


43 


of  its  position  and  shape,  however,  this  closed  oscillating  circuit  is 
&  poor  radiator  of  electrical  energy.  There  are  two  general  ways 
in  which  the  energy  of  this  circuit  can  be  transferred  to  the  antenna 
or  radiating  circuit;  or,  as  it  is  said,  two  ways  of  coupling  the  circuits. 
One  is  shown  in  figure  37,  where  the  ground  and  the  antenna  circuits 
are  shown  to  be  directly  connected  to  the  inductance  coil  of  the 
closed  circuit,  and  the  circuits  are  said  to  be  directly  connected, 
directly  coupled,  or  conductively  coupled.  The  coil  is  often  called 
the  antenna  coil  or  Jielix.  The  other  is  shown  in  figure  36,  where 
a  number  of  turns  in  the  coil  L2,  connected  between  the  antenna  and 
ground,  is  brought  near  enough  to  a  number  of  turns  of  the  coil  Lt 
in  the  closed  oscillating  circuit  to  have  oscillations  induced  in  the 
antenna  coil  and  circuit,  and  the  circuits  are  said  to  be  inductively 
coupled  or  connected.  The  two  coils  Lt  and  L2  form  an  oscillation 


FIG.  36. 

transformer,  as  it  is  usually  called,  the  coil  Lx  being  the  primary  and 
-coil  L2  the  secondary.  Hence  the  antenna  circuit  is  sometimes  called 
the  secondary  circuit.  There  is  no  essential  difference  in  the  opera- 
tion or  efficiency  of  the  transfer  of  energy  in  the  two  types  of  coup- 
ling, but  rather  that  each  may  have  advantages  in  certain  cases. 
In  direct  connected  sets  when  nearly  the  same  turns  are  connected 
in  both  the  primary  and  the  secondary  circuits — that  is,  when  most 
of  the  turns  in  use  are  common  to  both  circuits,  as  shown  in  figure 
38 — the  coupling  is  said  to  be  close  or  tight.  When  only  a  compara- 
tively few  turns  are  common  to  the  two  circuits,  as  shown  in  figure 
39,  the  coupling  is  said  to  be  loose.  Similarly  in  inductively  con- 
nected sets,  when  most  of  the  turns  in  use  in  the  two  circuits  are 
near  together,  as  when  one  coil  is  moved  inside  the  other,  as  shown  in 
figure  40,  the  coupling  is  close.  When  the  turns  in  use  are  not  near 


44 


RADIOTELEGRAPHY. 


together,  as  shown  in  figure  36,  the  coupling  is  loose.  In  the  case 
of  inductively  coupled  sets  it  is  evident  that  moving  the  coils  of  the. 
oscillation  transformer  nearer  together  will  tighten  the  coupling  or 
make  it  closer,  and,  vice  versa,  moving  the  coils  farther  apart  will 
loosen  the  coupling.  Similarly  in  the  directly  connected  sets,  if 
the  turns  in  use  in  either  circuit  are  moved  so  as  to  have  few  or 
even  no  turns  at  all  in  common  the  coupling  is  loosened,  as  shown  in 
figure  39.  The  coupling  may  be  made  loose  in  other  ways,  one  of 
which  is  illustrated  in  figure  41,  where  the  coil  L1.,,  often  known  as  a 


FIG.  37. 

loading  coil,  is  inserted  in  the  antenna  circuit,  thereby  adding  in- 
ductance not  coupled  with  the  primary  circuit.  Similarly  in  the  case 
of  inductively  connected  sets  the  coupling  may  be  loosened  by  in- 
serting the  loading  coil  L^  in  the  antenna  circuit,  as  shown  in  figure 
42.  In  both  these  cases  it  is  to  be  noted  that  the  result  is  practically 
the  same  as  though  the  turns  in  use  in  the  two  circuits  were  moved 
farther  apart  as  a  wiiole.  In  both  the  directly  connected  and  the 
inductively  connected  sets  the  coupling  may  also  be  loosened  by  in- 
serting a  loading  coil  in  the  primary  circuit,  as  shown  in  one 
case  in  figure  43.  By  means  of  these  loading  coils  a  directly  con- 


EADIOTELEGEAPHY. 


45 


nected  set  can  thus  be  made  as  loosely  coupled  for  practical  work 
as  an  inductively  connected  set.  In  such  a  circuit  as  that  in  figure 
41,  the  coil  which  is  common  to  both  circuits  and  serves  to  transfer 
the  energy  from  one  to  the  other  is  sometimes  called  the  coupling 
coil.  At  the  present  time  most  of  the  sets  in  use  in  the  Signal  Corps 
are  loosely  coupled  and  all  of  the  various  methods  *of  obtaining  loose 


FIG.  38. 

coupling  here  described  are  in  use,  each  one  having  advantages  in 
its  particular  radio  set. 

ANTENNA. 

The  open  or  radiating  circuit  has  its  own  natural  period  of  oscilla- 
tion expressed,  as  in  the  case  of  the  closed  circuit  mentioned  on 
page  19,  in  fraction  of  a  second.  We  can  impart  most  energy  to  it 
from  the  closed  oscillating  circuit  by  adjusting  the  inductance  or 
capacity,  or  both,  of  the  latter  until  the  oscillations  in  it  have  the 
same  frequency  as  in  the  open  circuit;  that  is,  until  the  two  circuits 
are  in  resonance.  Then  the  strongest  oscillations  or  the  greatest 
current  have  been  produced  in  the  antenna  as  shown  by  the  maximum 


46 


RADIOTELEGKAPHY. 


reading  in  a  hot-wire  ammeter  of  figures  38  to  43,  inclusive.  This 
ammeter  is  usually  connected  between  the  ground  and  the  secondary 
of  the  oscillation  transformer  but  may  be  connected  between  the  sec- 
ondary and  the  antenna. 

These  powerful  damped  high-frequency  oscillations  in  the  antenna 
or  open  circuit  produce  corresponding  periodic  disturbances  in  the 
surrounding  medium  which  spread  outward  in  the  form  of  electro- 
magnetic waves,  as  has  already  been  explained. 


I 
1 


FIG.  39. 

In  general  the  higher  the  antenna  the  greater  the  energy  in  the 
form  of  electromagnetic  waves  which  it  can  radiate  and  receive;  in 
other  words,  the  greater  the  distance  to  which  it  can  send  and  re- 
ceive signals.  In  most  cases  a  large  capacity  is  also  desired,  which 
can  be  secured  by  putting  up  a  number  of  vertical  wires,  but  there 
is  little  gain  in  capacity  unless  the  wires  are  at  least  a  foot  apart. 
Additional  capacity  and  increased  efficiency  in  radiation  can  be 
secured  by  using  a  flat  top  or  horizontal  spread  of  wires  at  the  top 


KADIOTELEGRAPHY. 


47 


of  the  mast,  which  becomes,  as  it  were,  one  plate  of  a  condenser,  the 
earth  being  the  other  plate  with  the  air  as  the  insulator.  Antennas 
are  often  divided  into  three  types,  depending  on  the  way  in  which  the 
wires  are  arranged  at  the  top,  such  as  umbrella,  inverted  L,  and  Tee, 
where  the  names  are  sufficiently  suggestive  so  as  not  to  require  a 
description.  The  umbrella  is  best  adapted  for  shore  stations  having 


FIG.  40. 

a  single  mast  or  tower  with  several  acres  of  land  around  the  station, 
and  has  been  largely  used  by  the  Signal  Corps. 

The  inverted  L  and  the  Tee  can  be  installed  on  shipboard  or  at 
shore  stations,  but  require  two  masts  or  towers.  In  the  case  of  the 
umbrella  antenna,  the  wires  extending  outward  from  the  mast  should 
be  kept  as  nearly  horizontal  as  possible  and  as  far  away  from  tree 
tops,  buildings,  roofs,  etc.,  as  circumstances  will  permit.  The  distant 
ends  are  dead-ended  at  high-potential  insulators  attached  to  long  guys 


48 


KADIOTELEGKAPHY. 


carried  out  to  stub  masts  or  deadmen.  These  guys  should  have 
insulators  inserted  every  50  or  100  feet  so  as  to  prevent  them  from 
serving  as  extensions  to  the  antenna  wires  and  thereby  bringing  the 
antenna  too  near  the  ground.  It  is  not  necessary  that  the  antenna 
wires  be  symmetrically  arranged  around  the  tower,  it  being  far  more 
important  that  advantage  be  taken  of  the  configuration  of  the  ground 
and  that  the  outer  ends  be  kept  well  elevated  than  that  a  symmetrical 
arrangement  be  made.  This  is  shown  in  the  plan  of  the  Signal  Corps 


I 
1 


FIG.  41. 

radio  installation  at  Fairbanks,  Alaska,  figure  44,  where,  on  account 
of  swampy  land  near  the  river  near  the  station,  a  symmetrical  ar- 
rangement is  practically  impossible. 

The  antenna  must  be  well  insulated,  particularly  at  the  outer  ends 
of  the  horizontal  wires,  as  otherwise  there  will  be  leakage  to  ground 
in  damp  weather  or  rainy  seasons,  which  will  cause  a  serious  loss  in 
efficiency  when  the  station  is  transmitting.  High-tension  insulators 
of  electrose  or  porcelain  are  usually  furnished  for  use  at  these  points 
of  the  circuit. 


EADIOTELEGEAPHY. 


49 


The  antenna  wires  are  generally  stranded,  thus  giving  somewhat 
greater  strength  than  a  solid  wire  of  the  same  weight.  For  perma- 
nent stations  a  phosphor-bronze  or  silicon-bronze  wire  is  generally 
used  consisting  of  seven  strands  of  either  No.  20  or  No.  14  B.  &  S. 
gauge,  and  for  the  portable  stations,  such  as  the  Signal  Corps  field- 
pack  sets,  an  antenna  cord  made  up  of  42  phosphor-bronze  wires 
stranded  around  a  hempcord  center.  A  very  low  resistance  in  the 


FIG.  42. 

antenna  wires  is  not  as  necessary  as  it  might  seem  to  be,  as  it  has 
been  shown  by  theory  and  proven  by  experiment  that  the  radiation 
of  electromagnetic  waves  introduces  a  resistance,  sometimes  called 
the  radiation  resistance,  which  in  general  is  many  times  the  high- 
frequency  resistance  of  the  wires  themselves.  This  radiation  resist- 
ance rarely  falls  below  2  ohms  on  a  ship  set  and  may  be  as  high 
as  20  or  30  ohms  in  a  shore  station.  When  the  antenna  resistance 
is  measured  under  service  conditions  it  includes  that  of  the  wires  at 

92448—15 4 


50 


RADIOTELEGRAPH!'. 


the  given  frequency,  the  resistance  of  the  ground,  and  that  due  to 
the  radiation  of  energy,  the  latter  being  generally  the  larger  part. 
A  typical  antenna  resistance  is  shown  in  figure  45,  where  it  is  to  be 
noted  that  the  resistance  is  largest  near  the  fundamental  wave  length 
of  the  antenna  and  is  smallest  at  a  wave  length  about  one  and  one- 
half  or  two  times  the  fundamental.  It  is  at  or  near  this  point  that 
many  stations  work  most  efficiently. 

ARTIFICIAL   ANTENNA. 

In  many  cases  it  is  convenient  to  make  station  tests  without  using 
the  actual  antenna,  particularly  where  such  use  would  cause  unnec- 


FIG.  43. 


essary  interference.  A  local  circuit  of  a  coil  L  and  condenser  T 
having  the  same  inductance  and  capacity  as  the  antenna,  and  called 
an  artificial  antenna,  is  often  used,  thus  serving  the  same  purpose 
as  an  artificial  line  or  cable  in  telegraph  tests.  When  a  resistance 
R  is  inserted  in  this  circuit  to  give  the  same  current  as  actually  flows 
la  the  antenna,  this  resistance  is  approximately  equal  to  the  antenna 
resistance  as  mentioned  on  page  49.  The  circuit  for  making  these 
measures  is  shown  in  figure  46,  where  the  circuit  of  L,  C\  and  R, 
which  replaces  the  antenna  when  the  switch  is  thrown  to  the  right, 
is  the  artificial  antenna. 


RADIOTELEGEAPHY. 


51 


The  antenna  inductance  L  and  capacity  C  can  be  easily  measured 
with  the  help  of  a  wave  meter  and  thus  a  suitable  coil  and  condenser 
selected  for  use  in  the  artificial  antenna  which  will  then  closely 


represent  the  actual  antenna.  First,  measure  the  fundamental  wave 
length  of  the  antenna  itself  ^  using  the  plain  Marconi  antenna  cir- 
cuit as  shown  in  figure  35.  Next  insect  a  loading  coil  of  known 


52 


RADIOTELEG  HAPH  Y. 


inductance  1  and  measure  the  fundamental  of  the  loaded  antenna  / 
Then  antenna  inductance  L  =    , '^milli 


and  antenna  capacity  0  =  0.000281 


A , .  „  «  ~j  microfarad . 


Thus  let  ^  =  430  meters,   y*2  =  980  meters,   and  1  =  0.145  millihenry. 
Hence 

185000X0.145      .ir, 
L=—     "~cn7A  -----  millihenry  =  0.0346  millihenry 


mf  =  0.0015  mf. 


0  =  0.000281 


Wave  length:— Meters 

FIG. 45. 

EFFICIENCY   OF   RADIO    SET. 

The  antenna  resistance,  the  radiation  resistance,  and  the  antenna 
current  all  change  as  the  frequency  or  wave  length  changes.  If  at 
any  one  frequency  or  wave  length,  the  square  of  the  antenna 
current  in  amperes  is  multiplied  by  the  antenna  resistance  in 
ohms,  the  product,  PR,  is  in  watts,  and  represents  the  power 
delivered  by  the  closed  oscillating  circuit  to  the  antenna;  that 
is,  it  is  the  antenna  input,  as  it  is  sometimes  called,  or  the  watts 
in  the  antenna.  If  the  number  of  watts  delivered  by  the  alternator  is 
known,  the  efficiency  from  alternator  to  antenna  can  be  found  by 
finding  the  quotient  of  watts  in  antenna  divided  by  the  watts  from 

„*  .  watts  in  antenna 

the  alternator,  thus  e  =  efficiency  =          -  ,  ,,  - 

J     watts  from  alternator. 

In  the  early  types  of  spark  sets  this  value  was  as  low  as  10  or  20  per 
cent,  whereas  in  modern  quenched  spark  sets,  it  may  be  as  high  as  50 


RADIOTELEGRAPHY. 


53 


per  cent  or  even  higher.  If  a  motor-generator  set  is  used  and  the 
number  of  watts  delivered  to  the  motor  is  known,  the  over-all  effi- 
ciency can  similarly  be  found  by  dividing  the  antenna  watts  by  the 

antenna  watts. 
motor  watts,  thus  over-all  «  = 


The  percentage  so  obtained  will  of  course  be  lower  than  before,  as  it 
allows  for  losses  in-  the  motor-generator  which  were  not  considered  in 
the  previous  case. 

The  rating  of  the  earlier  radio  sets  was  given  as  the  output  of  the 
alternator,  but  in  modern  sets  it  is  often  given  as  the  number  of  watts 
delivered  to  the  antenna.  In  the  latter  case  the  artificial  antenna 
may  be  used  and  its  inductance,  capacity,  resistance,  together  with 
the  current  and  watts  at  a  given 
wave  length  must  then  be  specified. 

When  steel  towers  are  used  they 
are  generally  heavily  insulated 
at  the  base,  but  provided  with 
switches  for  grounding  when  de-.  . 
sired,  as  during  lightning  storms, 
etc.  In  some  cases  the  station  be- 
comes more  efficient  in  transmitting 
if  the  tower  is  grounded.  In  gen- 
eral, however,  the  result  of  ground- 
ing can  be  told  only  by  tests  at  the 
receiving  station  of  the  loudness  of 
the  signals,  and  not  by  the  read- 
ings of  the  antenna  hot-wire  am- 
meter or  other  means  at  the  trans- 
mitting station.  The  grounding  of 
the  tower  generally  makes  it  nec- 
essary to  change  the  tuning  of  the 
transmitter,  and  there  are  corre-  FIG.  40. 

spending  changes  in  the  reading  of  the  antenna  ammeter,  but  in- 
creases in  its  reading  do  not  necessarily  mean  increases  in  the  signals 
at  the  receiving  station,  as  part  of  this  increase  is  due  to  increased 
flow  of  current  through  the  tower  to  ground.  It  is  for  this  reason 
that  the  results  of  grounding  should  always  be  tested  at  the  receiver. 

GROUND. 

An  efficient  ground  for  a  radio  station  is  very  different  from  that 
used  at  an  ordinary  telegraph  station.  The  latter  generally  has  a 
metal  plate  set  deep  in  wet  ground,  but  the  former  needs.  a  large 
spread  on  the  surface  or  just  under  it.  Thus,  instead  of  using  a  large 
copper  plate  or  rods  close  together,  a  far  better  type  of  ground 
would  be  to  use  wires  radiating  out  from  the  station,  or  to  duplicate 
the  umbrella  or  flat-top  antenna  system  a  short  distance  under  the 


54  EADIOTELEGEAPHY. 

surface  of  the  ground.  The  advantages  of  a  surface  ground  may 
be  understood  when  it  is  remembered  that  close  to  the  station  the 
magnetic  and  static  fields  are  very  intense,  so  that  if  they  had  to 
pass  down  through  the  earth  to  a  ground  plate  instead  of  being 
able  to  travel  wholly  on  the  surface,  as  shown  in  figure  9,  there 
would  be  introduced  an  additional  ground  resistance  and  local  earth 
currents  would  be  caused,  with  corresponding  losses.  The  use  of  a 
surface  ground  serves  to  reduce  these  losses  to  a  minimum.  It  should 
be  noted  that  the  instantaneous  values  of  the  transmitting  currents 
are  very  large  and  the  frequencies  very  high,  sometimes  a  million  or 
more  per  second,  so  that  considerable  copper,  such  as  stranded  wires 
or  copper  strip,  should  be  used  both  in  the  ground  wires  and  in  the 
leads  connecting  the  set  to  them. 

Another  type  of  ground  connection  which  has  been  successfully 
used  at  permanent  stations,  and  also  in  the  portable  field  sets,  is 
known  as  the  counterpoise.  In  the  permanent  stations  this  consists 
of  a  set  of  bare  horizontal  radial  or  parallel  wires,  which  are  sup- 
ported by  insulators  on  posts  7  feet  or  more  above  ground.  A 
counterpoise  of  a  fan  type  has  been  installed  at  Fort  Sam  Houston, 
Tex.,  in  which  bare  wires,  No.  10,  B.  &  S.  gauge,  190  feet  long,  ex- 
tend outward  from  the  station  under  the  antenna,  being  spaced  6 
feet  apart  at  the  station  and  20  feet  at  the  distance  ends.  A  counter- 
poise of  the  radial  type  has  been  installed  at  the  Fairbanks  (Alaska) 
station,  as  shown  in  figure  44,  where  the  wires  are  bare  hard-drawn 
copper  No.  12,  B.  &  S.,  about  210  feet  long,  and  spread  out  in  two 
arcs,  each  of  90  degrees.  A  counterpoise  is  particularly  efficient  in 
case  the  soil  is  very  dry,  as  at  Fort  Sam  Houston,  and  also  where 
there  is  a  heavy  snowfall,  as  at  Fairbanks.  At  the  latter  station  both 
a  ground  and  a  counterpoise  have  been  -installed.  In  the  case  of  the 
Signal  Corps  wagon  sets,  radial  counterpoise  wires  mounted  on  tem- 
porary poles,  carried  as  a  part  of  the  set,  were  used  at  first,  but  now 
have  been  replaced  by  the  same  type  as  that  of  the  pack  sets,  which 
consist  of  rubber-covered  wires,  each  100  feet  long,  laid  out  radially 
on  the  ground.  Although  not  directly  connected  with  the  ground  at 
all,  these  wires  really  constitute  one  plate  of  a  condenser,  the  ground 
being  the  other. 

WAVE    LENGTHS. 

Before  describing  the  various  receiving  circuits  and  the  theory  of 
their  operation,  some  of  the  terms  applied  to  them  and  to  the  trans- 
mitting circuits  will  be  defined. 

In  the  mechanical  illustrations  of  damped  oscillations  and  res- 
onance, by  means  of  the  steel  spring  and  the  tuning  forks  it  was 
convenient  to  use  both  the  frequency  expressed  in  the  number  of 
oscillations  per  second  and  the  period  expressed  in  fractions  of  a 
second.  The  same  terms  were  used  in  describing  the  electrical  oscilla- 


RADIOTELEGRAPH  Y.  55 

tions  in  the  radio  circuits,  and  although  this  usage  is  entirely  correct, 
it  is  somewhat  more  common  to  use  the  term  wave  length,  which  will 
be  defined  in  the  following  paragraphs. 

If  at  any  instant  an  electromagnetic  wave  begins  to  radiate  from 
an  antenna,  at  the  end  of  one  second  of  time  the  wave  will  have 
reached  a  point  300,000,000  meters  distant;  that  is,  it  is  said  that  its 
velocity  is  300,000,000  meters  per  second,  or,  as  it  is  often  abbreviated, 
V  =  300,000,  000  meters.  During  this  interval  of  time  the  direction 
of  the  magnetic  and  the  static  lines  of  the  wave  has  been  reversed  very 
many  times  ;  in  fact,  as  many  times  as  the  oscillations  in  the  antenna 
have  been  reversed.  Similarly  in  this  interval  of  space  both  fields 
will  be  in  the  same  direction  at  very  many  points,  all  separated  by 
equal  distances,  as  represented  in  figure  9.  The  distance  between 
any  two  such  points  is  called  a  wave  length  and  is  generally  given  in 
meters,  the  symbol  for  which  is  L 

It  is  evident  that  the  greater  the  number  of  times  per  second  that 
the  two  fields  have  been  reversed  the  shorter  will  be  the  distance  in 
meters  between  the  points  where  the  fields  are  in  the  same  direction; 
that  is,  the  shorter  the  wave  length;  and,  vice  versa,  the  fewer  the 
number  of  times  per  second  that  the  fields  have  been  reversed  the 
longer  will  be  the  distance  between  the  points  where  the  fields  are 
in  the  same  direction;  that  is,  the  longer  will  be  the  wave  length. 
If  N  is  the  number  of  points  in  the  distance  300,000,000  meters  that 
the  fields  have  the  same  direction,  and  if  X  is  the  wave  length  in 
meters,  then  we  have  the  relation  NxA  =  V.  This  is  one  of  the 
fundamental  relations  in  radiotelegraphy.  This  may  be  shown 
graphically  in  figure  47,  where  to  secure  simplicity  only  the  static 
field  is  indicated,  in  which  it  is  seen  that  the  direction  of  the  field  is 
repeated  N  times  in  the  distance  V  =  300,000,000  meters,  which  is 
traveled  in  one  second  of  time. 

A  short  table  of  wave  lengths  and  frequencies,  computed  from  the 
equation  NxA  =  V,  is  given  below: 


Wave  Frequency  in 

length  in  oscillations 

meters.  per  second. 

100  ..............................................  3,  000,  000 

200  ...............................................  1,  500,  000 

300  ...............................................  1,  000,  000 

400  ...............................................  750,  000 

500  ...............................................  600,  000 

600  ...............................................  500,  000 

1,  000  ...............................................  300,  000 

2,  000  ...............................................  150,  000 

3,  000  ...............................................  100,  000 

4,  000  ..............  ................................  75,  000 

5,  000  ...........................  ....................  60,  000 

6,  000  ...............................................  50,  000 

10,  000  ...............................................  30,  000 


56 


RADIOTELEGRAPH  V. 


(VI 


From  this  table  and  from  the  relation  T  =  ^  given  on  page  26, 

it  is  seen  that  the  shorter  the  wave  length  the  higher  is  the  fre- 
quency in  number  of 
A     A          oscillations  per  second 
I  and    the    shorter    the 

period  of  each  oscil- 
lation in  fractions  of 
a  second;  and,  vice 
versa,  the  longer  the 
wave  length  the 
lower  is  the  fre- 
quency in  oscillations 
per  second  and  the 
longer  the  period  of 
each  oscillation  in 
fractions  of  a  second. 
Although  the  wave 
length  is  rarely,  if 
^  ever,  measured  as 

~Y       §  01         ^ne  distance  in  space 
between     two    points 


i 

*9> 


§§ 

fc   ^ 


_i 


Q> 


l§ 

K    CT) 


<0  ^  ^  where  the  electro- 
iT  §  2  magnetic  fields  have 
^  the  same  direction, 
yet  it  can  be  very 
accurately  measured 
by  other  means.' 
One  of  these 
makes  use  of  the 
relation  N X ^  =  V, 
and  may  be  briefly 
described  as  follows. 
It  consists  in  pho- 
tographing the  dis- 
charges in  a  wave 
train  at  the  spark 
gap  on  a  sensitive 
plate  which  is  moved 
past  the  gap  at  a 
very  rapid  but 
known  speed.  From 
the  speed  of  the 
plate  and  separation 


T 


I._L1 


of  the  successive  images  it  is  possible  to  determine  the  frequency 
that  is,  N — and  hence  the  wave  length  L 


RADIOTELEGRAPH  Y.  57 

FREQUENCIES   IN   RADIO   MEASUREMENTS. 

It  will  be  noted  that  it  has  been  necessary  to  speak  of  the  frequency 
of  circuits  from  two  or  three  different  points  of  view,  which  will  be 
summarized  as  follows:  (1)  The  frequency  of  the  alternator,  which 
depends  upon  the  speed  and  design  of  the  machine,  as  from  60  to  500 
cycles  per  second.  This  frequency  is  independent  of  all  of  the  radio 
circuits.  (2)  The  spark  frequency  or  wave-train  frequency,  which 
depends  on  the  alternator  frequency,  the  capacity  of  the  closed- 
circuit  condenser,  the  voltage  at  the  spark  gap,  etc.,  as  120  to  500  or 
1,000  sparks  or  wave  trains  per  second.  (3)  The  frequency  of  the 
oscillations  in  the  radio  circuits,  which  depends  only  on  the  capacity 
and  inductance  in  the  circuit  in  question,  as  1,000,000  oscillations  per 
second  for  a  wave  length  of  300  meters,  or  100,000  oscillations  per 
second  for  a  wave  length  of  3,000  meters.  Use  must  be  made  of  all  of 
these  frequencies  in  dealing  with  the  problems  of  radiotelegraphy. 

WAVE   METER. 

The  instrument  used  to  measure  the  wave  length  of  the  oscilla- 
tions, and  hence  the  frequency  or  period  as  may  be  desired,  is  called 
a  wave  meter.  It  consists  essentially  of  a  closed  circuit  of  coil  and 
condenser,  from  the  known  inductance  and  capacity  of  which  the 
frequency  or  wave  length  can  be  computed  by  the  formulas 


where  the  inductance  L  and  the  capacity  C  must  be  expressed  in  the 
units  of  the  electromagnetic  system,  and  h  will  be  in  meters  if  V  is  in 
meters,  or  V  =  300,000,000.  As  it  may  be  sometimes  more  convenient 
to  use  the  units  of  the  practical  system,  as  microfarads  and  milli- 
henrys,  for  example,  the  formulas  will  also  be  given  for  these  units 
as  follows: 

AT      5033       5000 

=        =        aPProxlmately 


A  =  59600  VETO  -  60000  VI^C  approximately. 

Thus  if  L  is  0.0352  millihenrys  and  C  is  0.0020  mf  .,  L  X  C  is  0.0000704, 
v/0.0000704  is  0.00839,  and  A  is  59600  X  0.00839  meters,  or  is  500 

5033 
meters.     Similarly   N   is  -  =  600,000    oscillations    per    second, 

.UUooU 

which  agrees  with  the  value  in  the  table  on  page  55  for  a  wave  length 
of  500  meters. 

In  order  to  include  a  wide  range  of  wave  lengths  or  frequencies 
several  coils  are  generally  provided,  which,  in  the  best  meters,  are 
wound  with  litzendraht,  thereby  to  make  the  high  frequency  resist- 
ance low,  and  hence  the  meter  sensitive  and  the  tuning  sharp.  The 
variable  condenser  has  either  air  or  oil  for  the  dielectric  rather  than 


58 


KADIOTELEGRAPHY. 


a  solid  material,  so  that  there  is  little  or  no  internal  losses.  By 
means  of  the  variable  condenser  the  circuit  can  be  tuned  to  resonance 
with  any  circuit  whose  wave  length  is  desired.  To  indicate  resonance 
a  hot-wire  ammeter  or  wattmeter  may  be  used,  with  a  suitable  shunt 
to  keep  the  resistance  in  circuit  low,  as  shown  in  figure  48,  where  C 
is  the  variable  condenser,  L  the  inductance,  and  A  the  shunted  am- 
meter or  wattmeter.  To  measure  the  wave  length  the  wave  meter  is 


:c 


FIG.  48. 


brought  near  the  circuit  in  question,  but  loosely  coupled  with  it,  and 
the  capacity  of  the  condenser  is  varied  until  a  setting  is  found  that 
gives  a  maximum  reading  in  the  hot-wire  meter.  From  this  setting 
and  the  calibration  of  the  instrument  the  wave  length  can  be  found. 
In  some  cases  meters  are  provided  with  a  tube  previously  filled  with 
a  gas,  such  as  helium  or  neon,  and  then  partially  exhausted,  to  be 
connected  across  the  terminals  of  the  condenser  to  indicate  resonance. 
When  the  meter  is  in  resonance  there  is  a  maximum  current  flowing 


FIG.  49. 

in  its  circuit,  and  this  produces  a  maximum  voltage  across  the  con- 
denser terminals.  This  potential  causes  a  very  small  current  to  flow 
through  the  gas,  which  is  lighted  up  thereby,  and  thus  indicates  the 
setting  for  resonance  from  which  the  wave  length  can  be  found  as 
before.  In  other  cases  it  is  convenient  to  use  a  detector  to  indicate 
resonance,  in  which  case  the  meter  becomes  a  receiving  set  with  tele- 
phones, etc.,  as  shown  in  figure  49,  where,  as  before,  C  and  L  are  the 


KADIOTELEGRAPHY. 


59 


capacity  and  inductance,  D  the  detector,  T  the  telephones,  etc.  The 
setting  of  the  condenser  where  the  signals  are  loudest  is  the  resonance 
point,  from  which  the  wave  length  can  be  obtained  as  before.  In  a 
few  cases  a  receiving  circuit  such  as  that  shown  in  figure  50  is  used, 
which  from  the  character  of  the  circuit  to  the  detector  is  sometimes 
called  a  unipolar  connection.  The  explanation  of  its  operation  is 
that  when  the  current  circulates  in  the  wave  meter  itself  there  is  cur- 
rent enough  sent  along  the  short  wire  to  operate  the  detector  and 
telephones. 

In  addition  to  these  uses  of  the  wave  meter  at  a  transmitting  sta- 
tion there  are  other  equally  important  ones  at  a  receiving  station 
which  will  be  described  under  the  subject  of  receivers. 

NATURAL  OB  FUNDAMENTAL  WAVE  LENGTH,  FREQUENCY,  AND 

PERIODS. 

One  of  the  simplest  and  at  the  same  time  one  of  the  most  important 
uses  of  a  wave  meter  at  a  transmitting  station  is  in  the  measurement 


FIG.  50. 


of  the  fundamental  wave  length  of  an  antenna,  which  will  be  described 
next.  It  has  been  stated  that  a  circuit  having  a  capacity  C  and  induc- 
tance L  has  a  wave  length  A  =  2  mj-jL  C,  a  frequency  N  =  o 


and  a  period  T  =  ^T  =  2  Tr-y/L  C.  These  values  are  generally  called,  re- 
spectively, the  natural  or  fundamental  wave  length,  frequency,  and 
period.  These  terms  apply  to  an  antenna  as  well  as  to  a  closed  cir- 
cuit. Although  the  antenna  has  no  coil  or  condenser  in  its  cir- 
cuit, the  inductance  is  distributed  along  the  length  of  the  wire,  as  is 
the  capacity.  In  such  a  circuit  it  is  said  that  there  is  distributed 
inductance  and  distributed  capacity,  as  distinguished  from  the  con- 
centrated or  lumped  inductance  and  lumped  capacity  in  a  coil  and 
condenser  of  a  local  circuit.  Theory  and  experiment  have  shown  that 
a  single  vertical  wire  of  length  L  has  a  natural  wave  length  of  about 
4  times  its  length;  that  is,  the  fundamental  wave  length  is  approxi- 


60  RADIOTELEGRAPH  Y. 

mately  4  L.     Thus,  a  wire  100  feet  long  will  give  a  fundamental  wave 
length  of  400  feet;  that  is,  about  122  meters,  in  even  numbers. 

1  inch=2.54  centimeters. 
1  foot=30.48  centimeters. 
100  feet=3,048  centimeters =30. 48  meters. 
400  feet=122  meters. 

If  the  single- wire  antenna  is  of  the  inverted  "L"  type  or  is  hori- 
zontal, the  fundamental  wave  length  will  be  increased  to  between 
4  L  and  5  L.  If  there  are  several  wires  in  the  antenna  these  simple 
relations  do  not  apply  and  the  fundamental  wave  length  must  be 
measured  by  a  wave  meter. 

The  plain  Marconi  antenna,  shown  in  figure  35,  is  one  of  the  sim- 
plest circuits  for  the  measurement  of  the  fundamental  wave  length 
of  an  antenna. 

A  single  turn  of  wire  4  or  5  inches  in  diameter  is  often  inserted 
in  the  antenna  near  the  ground  where  the  potential  is  low,  which 
serves  as  a  convenient  means  of  coupling  the  wave  meter  to  the 
antenna.  The  insertion  of  such  a  small  turn  has  no  appreciable  effect 
on  the  fundamental  wave  length,  and  in  many  stations  it  forms  a 
permanent  part  of  the  antenna. 

The  fundamental  wave  length  of  an  antenna  in  small-sized  sets, 
as  in  field  sets  or  on  artillery  tugs,  may  be  as  short  as  200  to  250 
meters,  and  in  large-sized  sets  may  be  as  long  as  1,500  to  2,000  meters. 
In  general  the  longer  the  antenna  wires,  the  higher  the  masts,  and 
the  greater  the  number  of  the  wires  the  longer  is  the  fundamental 
wave  length. 

In  the  circuits  shown  in  figures  36-43,  illustrating  some  of  the 
common  types  of  transmitting  circuits,  it  will  be  noted  that  a  coil 
has  always  been  inserted  in  series  between  the  antenna  and  ground. 
The  insertion  of  such  an  inductance  always  increases  the  wave  length 
of  the  circuit.  Thus  the  fundamental  wave  length  of  a  certain 
antenna  alone  may  be  300  meters;  an  antenna  coil  of  inductance  of 
0.12  millihenry  is  inserted  and  the  wave  length  of  the  circuit,  antemia- 
coil-ground,  has  now  been  increased  to  about  600  meters.  It  is  evi- 
dent then  that  none  of  the  transmitting  sets  of  figures  36-43  can 
radiate  a  wave  length  shorter  than  the  fundamental  wave  length  of 
the  antenna  itself.  Inasmuch  as  both  ships  and  shore  stations  must 
be  prepared  to  use  a  wave  length  of  300  meters  according  to  the  regu- 
lations of  the  International  Radio  Telegraph  Convention  it  is  evident 
that  a  series  condenser  .must  be  inserted  in  some  cases  as  shown  in 
figure  51. 

The  insertion  of  such  a  condenser  always  shortens  the  wave  length 
of  the  circuit.  Thus,  if  an  antenna  installed  on  a  ship  was  found 
to  have  a  fundamental  wave  length  of  450  meters  and  it  became 
necessary  to  use  a  wave  length  of  300  meters,  a  coil  must  be  inserted 


RADIOTELEGKAPHY. 


61 


in  the  antenna  circuit  to  permit  it  to  be  coupled  to  the  closed  circuit, 
which  would  lengthen  the  wave  length  somewhat,  and  then  a  series 
condenser  must  be  inserted  to  bring  the  wave  length  of  the  circuit 
antenna-coil-condenser-ground  to  300  meters.  Such  a  condenser 
should  be  used  only  when  it  is  absolutely  necessary,  as  it  is  generally 
subjected  to  high  potentials  which  give  brush  discharges  and  con- 
sequent losses.  In  many  cases  it  is  better  to  install  a  second  and 
smaller  antenna  having  a  fundamental  wave  length  sufficiently  short 
for  the  purpose  in  question.  This  has  often  been  done  both  on  ships 
and  at  shore  stations.  When  transmitting  on  the  short  antenna, 
the  station  end  of  the  large  antenna  should  be  left  insulated,  and,  vice 
versa,  when  transmitting  on  the  large  antenna,  the  short  antenna 
should  be  left  insulated  or  else  connected  into 
circuit  as  part  of  the  large  antenna. 

TUNING   OF   TRANSMITTING    SETS. 
MECHANICAL   ILLUSTRATION    OF    COUPLING. 

Before  describing  the  methods  of  tuning  the 
various  types  of  transmitters  and  the  measure- 
ment of  the  radiated  wave  lengths,  some  mention 
must  be  made  of  coupling  and  its  effects  on  the 
tuning  of  circuits. 

The  theory  of  coupled  circuits,  including  that 
of  the  quenched-spark  transmitter,  can  be  simply 
illustrated  by  a  mechanical  model  consisting  of 
two  equal  weights  suspended  by  two  equal  lengths 
of  string  from  points  on  a  slightly  stret  bed 
string,  as  show^n  in  figure  52.  If  the  weight  P 
is  pulled  to  one  side  and  released  it  will  exe- 
cute a  series  of  damped  oscillations  (correspond- 
ing to  the  charging  and  -the  oscillatory  discharg- 
ing of  the  primary  or  closed-circuit  condenser).  On  account  of 
movements  of  the  stretched  string  (corresponding  to  the  coup- 
ling) this  soon  causes  the  weight  S  to  begin  oscillating  (cor- 
responding to  the  induced  oscillations  in  the  secondary  circuit), 
and  in  a  short  time  it  will  be  oscillating  very  nearly  as  much 
as  P  had  been  doing,  but  in  the  meantime  P  has  practically 
stopped  oscillating.  In  a  short  time,  however,  P  will  again  be 
oscillating  nearly  as  much  as  before,  but  S  will  have  stopped.  Thus 
it  is  seen  that  the  energy  is  first  in  one  oscillating  weight  and  then  in 
the  other,  or  that  there  is  a  transfer  of  energy  back  and  forth  from 
one  to  the  other.  This  exchange  will  continue  until  the  energy  is  all 
wasted  or  used  up  in  friction,  etc.  This  can  be  represented  as  in 
figure  53,  where  the  upper  and  lower  curves  correspond  respectively 
to  the  oscillations  of  the  weights  P  and  S.  It  will  be  noted  that 


u 


FIG.  6 


62 


EADIOTELEGKAPHY. 


in  both  curves  of  figure  53  the  amplitudes  do  not  die  down  steadily 
toward  zero,  but  rather  through  a  series  of  maximum  and  minimum 
values.  Whenever  such  a  series  of  maximum  and  mimimum  values 
occur,  sometimes  called  beats,  it  can  be  shown  by  theory  that  it  is  due 


FIG.  52. 


to  the  fact  that  each  weight  is  oscillating  successively  at  two  slightly 
different  rates  or  frequencies,  one  being  slightly  slower  and  the  other 
slightly  faster  than  its  normal  rate;  that  is,  when  not  coupled  with 
the  other  weight.  In  general,  it  will  be  found  that  the  less  the  move- 
ment of  the  horizontal  string  (corresponding  to  loose  coupling)  the 


FIG.  53. 


less  frequent  will  be  the  transfer  of  energy  from  one  weight  to  the 
other,  and,  vice  versa,  the  greater  the  movement  of  this  string  (cor- 
responding to  close  coupling)  the  more  frequent  will  be  the  transfer. 
If  two  circuits,  one  of  which  contains  a  spark  gap,  are  separately 
tuned  to  the  same  frequency  or  wave  length  by  means  of  a  wave 


RADIOTELEGRAPHY. 


63 


meter  and  then  are  very  loosely  coupled,  it  will  be  found  that  there 
can  be  detected  only  one  wave  length  in  each,  which  is  the  same  as 
that  to  which  they  were  independently  adjusted  at  first,  as,  for 
example,  as  shown  by  the  curve  with  the  single  hump  at  300  meters 
in  figure  54.  When,  however,  the  coupling  has  been  somewhat 
increased  or  made  tighter  it  will  be  found  that  now  there  are  two  wave 
lengths  in  each  circuit,  one  of  which  is  shorter  and  the  other  longer 
than  that  to  which  the  circuits  were  tuned  at  first.  At  this  coupling 
no  readjustment  of  the  tuning  of  the  circuits  can  be  made  which  will 


200 


300 

Wave  Length:— Meters 


400 


FIG.  54. 


give  a  single  wave  in  both  of  the  same  length  as  before.  If  the 
coupling  is  still  farther  increased,  the  two  wave  lengths  will  be  sepa- 
rated still  farther  from  the  single  value  first  measured.  If  the  circuit 
containing  the  spark  gap  is  the  closed  oscillatory  or  primary  circuit 
of  a  transmitter  and  the  other  circuit  is  the  open  or  radiating  circuit, 
then  it  is  evident  that  two  wave  lengths  will  be  radiated  as  shown  in 
figure  54,  one  at  a  wave  length  of  275  meters  and  the  other  at  a  wave 
length  of  330  meters. 

The  two  very  loosely  coupled  circuits  with  the  same  wave  length 
in  each  correspond  to  the  case  of  a  very  small  motion  of  the  string 


64  RADIOTELEGRAFHY. 

with  a  single  transfer  of  energy  from  one  weight  to  the  other/  The 
closely  coupled  circuits  with  two  wave  lengths  in  each  correspond 
to  the  case  of  a  large  motion  of  the  string  with  frequent  transfers 
between  the  two  weights.  In  other  words  in  very  loosely  coupled 
circuits  the  normal  frequency  or  wave  length  of  each  is  unchanged, 
and  only  one  wave  length  can  be  detected  in  both.  On  the  other 
hand,  in  closely  coupled  circuits  the  normal  frequency  or  wave  length 
of  each  is  changed,  being  made  slower  (longer  wave  length)  and 
then  faster  (shorter  wave  length),  so  that  oscillations  are  taking 
place  successively  at  two  wave  lengths  as  shown  bv  the  wave  meter 
in  figure  54.  The  existence  of  the  two  frequencies  is  thus  due  to  the 
transferring  of  the  energy  back  and  forth  between  the  two  circuits, 
the  disadvantages  of  which  will  be  mentioned  in  the  description  of 
quenched  spark  sets. 

TUNING  WITHOUT  WAVE  METER. 

The  circuits  of  a  directly  or  inductively  coupled  set  using  the 
ordinary  type  of  open  spark  gap  can  be  tuned  to  resonance,  either 
with  or  without  the  help  of  a  wave  meter,  but  the  meter  should  be 
used  whenever  possible.  If  no  wave  meter  is  available  the  adjust- 
ments can  be  made  as  follows :  Insert  several  turns  of  inductance  in 
the  open  or  antenna  circuit,  a  few  in  the  closed  circuit,  and  note 
the  antenna  ammeter  reading.  Change  the  number  of  turns  in  the 
closed  circuit  and  also  the  coupling  if  necessary  until  a  maximum 
reading  is  obtained  in  the  ammeter.  Make  a  record  of  these  best 
adjustments — the  number  of  "turns  in  each  circuit,  the  coupling,  and 
antenna  ammeter  reading.  Next,  using  a  different  number  of  turns 
in  the  open  circuit,  repeat  until  the  best  adjustment  is  obtained  under 
these  conditions,  and  make  a  record  of  these  readings,  etc.  If  there 
is  no  ammeter  in  the  antenna  circuit  a  spark  gap  can  be  connected 
in  parallel  with  the  inductance  coil  in  the  antenna;  that  is,  between 
the  antenna  and  ground  and  the  circuits  adjusted  until  the  longest 
possible  spark  is  obtained,  in  which  case  the  circuits  are  in  resonance 
as  before.  The  ammeter  indicates  when  the  current  in  the  antenna 
is  a  maximum  and  the  gap  when  the  potential  at  the  antenna  is  a 
.maximum,  both  of  which  are  conditions  of  resonance.  These  are 
the  simplest  methods,  but  not  the  best.  The  adjustments  should  be 
made  with  a  wave  meter  for  reasons  that  will  be  made  clear  in  the 
following  paragraphs. 

TUNING    WITH    WAVE    METER. 

When  a  wave  meter  is  available  the  wave  lengths  of  the  closed  cir- 
cuit, uncoupled  from  the  open  circuit,  should  be  measured  for  dif- 
ferent numbers  of  turns  in  the  closed  circuit  or  primary  coil  and  the 
results  plotted  as  shown  in  figure  55.  Next,  using  the  antenna  and 
the  open  circuit  inductance  coil  as  a  plain  Marconi  antenna  similar 


KADIOTELEGKAPHY. 


65 


to  that  shown  in  figure  35,  measure  the  wave  lengths  for  different 
numbers  of  turns  and  plot  the  results  as  shown  in  the  figure.  It  will 
be  seen  that  when  4  turns  are  in  the  primary  circuit  and  9  in  the 
open,  both  are  tuned  to  a  wave  length  of  600  meters.  If  these  turns 
were  used  in  a  directly  connected  or  directly  coupled  set  with  a  single 
coil,  as  shown  in  figure  38,  and  the  coupling,  etc.,  adjusted  to  give  a 
maximum  antenna  ammeter  reading  as  described  in  the  previous 
paragraph,  it  will  almost  certainly  be  found  that  when  the  radiated 
wave  length  is  tested  with  a  wave  meter,  loosely  coupled  with  a  single 
turn  in  the  antenna  or  ground  circuit,  as  mentioned  on  page  60,  there 
will  be  two  wave  lengths  or  two  humps,  as  they  are  often  called,  such 
as  are  shown  in  figure  54.  These  two  wave  lengths  are  caused  by  too 


1400 


1200 


1000 


I 
^600 


^400 

I 


200 


4-  6 

Turns 


20 


FIG.  55. 


close  coupling  between  the  circuits.  They  can  not  be  shown  by  the 
antenna  ammeter,  which  gives  the  sum  of  the  currents  flowing  in  the 
circuit  without  regard  to  the  wave  lengths,  but  will  always  be  shown 
by  a  wave  meter,  which  gives  the  strengths  of  the  currents  at  the 
different  wave  lengths.  In  many  cases  it  will  be  found  that  when  the 
coupling  is  loosened  and  the  circuits  slightly  retuned  the  antenna 
ammeter  reading  may  be  reduced,  but  the  wave  meter  reading  will 
show  only  a  single  wave  length  or  hump  and  the  current  at  that  value 
will  be  much  larger  than  with  the  previous  adjustment.  As  the  wave 
meter  circuit  corresponds  very  closely  to  the  receiving  circuit  at  the 
distant  station,  it  will  almost  certainly  be  found  that  the  adjustment 
to  a  single  wave  length  will  give  louder  signals  than  the  other  ad  jus  t- 
92448—15 5 


66  RADIOTELEGRAPHY. 

ment.  Thus  in  figure  54  are  shown  two  curves  taken  from  an  actual 
transmitter,  the  one  with  the  double  hump  when  the  circuits  were  tuned 
to  a  maximum  antenna  ammeter  reading  and  the  other  when  the  cir- 
cuits were  more  loosely  coupled  and  adjusted  to  a  single  wave  length 
with  a  wave  meter.  The  antenna  ammeter  leading  was  less  in  the 
second  case  than  in  the  first,  but  the  wave  me.ter  test  and  a  receiving 
test  at  the  other  station  showed  that  much  was  gained  both  in  loud- 
ness  of  signals  and  in  sharpness  of  tuning.  Although  in  some  cases 
it  may  be  possible  to  radiate  more  energy  with  the  double  wave 
lengths,  yet  not  always  will  the  signals  be  louder,  for  the  reason  that 
most  receiving  sets  can  be  adjusted  to  receive  only  one  wave  length 
at  any  one  adjustment  and  all  energy  at  other  wave  lengths  or  in 
other  humps  is  wasted  as  far  as  this  receiver  is  concerned.  In  a  very 
few  cases  receivers  have  been  designed  to  receive  at  two  wave  lengths 
or  humps  at  the  same  time,  in  which  case  the  second  wave  length  will 
not  be  wasted ;  but  such  receivers  have  the  disadvantage  of  being  sub- 
ject to  interference  on  both  wave  lengths. 

There  is  a  most  serious  objection  to  the  use  of  transmitters  with 
double  wave  lengths,  or  humps,  on  account  of  the  interference  which 
they  cause.  Thus  in  figure  54  it  is  seen  that  this  transmitter  is  send- 
ing out  signals  on  275  and  325  meters  wave  lengths  and  is  preventing 
another  station  from  working  on  either  wave  length,  whereas  if  prop- 
erly tuned  as  at  300  meters  the  interference  is  reduced  to  one  wave 
length.  It  is  for  this  reason  that  legislation  has  been  enacted  pro- 
hibiting the  operation  of  a  station  with  two  such  humps.  The  law 
permits  the  use  of  the  double  hump  when  one  is  not  greater  than  one- 
tenth  of  the  other  as  tested  in  a  wave  meter.  There  are  further  re- 
strictions about  the  larger  of  the  two  humps,  or  about  a  single  hump 
in  case  only  one  is  found.  It  must  not  be  broad  or  flat  topped,  mean- 
ing that  the  oscillations  in  the  antenna  can  not  be  highly  damped,  as 
is  the  case  of  the  plain  Marconi  antenna  of  figure  35.  A  measure  of 
the  damping  is  prescribed  which  must  not  be  exceeded.  This  measure 
is  called  the  logarithmic  decrement  and  is  described  on  page  120 
under  the  subject  of  "  Measures  of  damping." 

THEORY  OF  OPERATION  OF  QTJENCHED-GAP  TRANSMITTER. 

Most  of  the  sets  now  supplied  by  the  Signal  Corps  are  of  the 
quenched-gap  type,  and  a  brief  outline  of  the  theory  of  its  operation 
will  be  given. 

If  in  a  quenched-spark  transmitter,  with  its  circuits  correctly  ad- 
justed to  radiate  a  single  sharply  defined  wave  length,  the  gap  is  re- 
placed by  an  ordinary  type  of  opsn  gap,  it  will  be  found  by  a  wave 
metei  test  that  there  are  now  two  wave  lengths.  This  shows  that  the 
single  wave  length  was  not  secured  by  an  adjustment  of  a  very  loose 
coupling  between  the  circuits,  but  rather  by  a  property  of  the 
quenched  gap  itself.  An  explanation  of  the  action  of  the  gap  can  be 


RADIOTELEGRAPH  Y.  07 

made  by  reference  to  figure  53,  where  it  will  be  noted  that  near  the 
point  marked  "Q"  in  the  upper  curve,  the  amplitude  of  the  primary 
current  has  reached  its  first  minimum  value  in  the  course  of  the  beats 
mentioned  on  page  62.  On  account  of  the  strong  cooling  action  of 
the  gap,  due  to  the  use  of  the  cooling  flanges  and  the  blower,  the  spark 
is  quenched  or  stopped  at  this  point  in  the  wave  train  and  the  primary 
circuit  is  thus  opened.  When  proper  cooling  is  provided  the  spark 
can  not  be  started  again  in  this  wave  train  and  the  gap  is  not  broken 
down  until  the  next  alternation.  At  the  same  tune  that  the  primary 
current  is  a  minimum  it  will  be  noted  that  the  secondary  current  is  a 
maximum ;  that  is,  practically  all  the  energy  is  located  in  the  second- 
ary circuit.  As  the  primary  circuit  has  now  been  opened  so  that  there 
can  be  no  transfer  of  energy  back  to  it,  all  is  retained  in  the  second- 
ary where  it  is  available  for  radiation.  As  a  result  there  are  no  beats 


FIG.  56. 

in  the  secondary,  the  oscillations  in  it  persist  for  a  longer  tune,  and 
more  energy  is  radiated.  This  is  shown  hi  figure  56,  where  the  pri- 
mary current  has  been  stopped  at  the  point  corresponding  to  Q  of 
figure  53  and  the  secondary  continues  to  oscillate  as  shown.  When- 
ever there  is  a  transfer  of  energy  back  to  the  primary  where  it  is  not 
available  for  radiation  there  are  losses  due  to  heating,  etc.,  and  so  less 
energy  is  left  for  radiation  than  if  there  had  been  no  such  transfer. 
The  quenched  spark  transmitter  has  then  two  advantages  over  a  trans- 
mitter with  the  ordinary  type  of  open  gap — greater  efficiency  and  the 
radiation  of  more  sharply  defined  wave  lengths. 

When  the  adjustments  of  a  quenched  spark  transmitter  have  been 
correctly  made — that  is,  the  circuits  are  in  resonance,  the  coupling 
is  right,  etc. — a  simple  experiment  will  show  that  the  primary  cur- 
rent is  a  minimum;  that  is,  the  spark  has  been  quenched  and  the 


68 


EADIOTELEGKAPHY. 


primary  current  has  been  stopped  quickly,  as  at  the  point  Q  of 
.figure  53,  and  that  at  the  same  time  the  secondary  current  is  a 
maximum;  that  is,  it  persists  for  a  long  time,  as  shown  in  figure  56. 
The  experiment  consists  in  making  simultaneous  readings  of  the 
•currents  in  the  primary  and  secondary  oscillating  circuits  and  plot- 
ting the  readings  for  the  different  separations  or  couplings  of  the 
primary  and  secondary  coils.  This  is  shown  in  figure  57,  where  the 
scale  at  left  is  in  amperes  and  that  at  the  bottom  is  the  separation  of 
the  two  coils,  the  upper  curve  being  for  the  primary  and  the  lower 
for  the  secondary.  At  the  point  of  correct  coupling  the  primary 
current  was  a  minimum  and  the  secondary  or  antenna  current  a 
maximum. 

From  these  curves  it  will  be  seen  that  the  coupling  of  the  two 
circuits  of  a  quenched-spark  transmitter  is  a  very  important  and 

critical  adjustment,  upon  the  cor- 
rect value  of  which  the  efficiency 
is  largely  dependent. 

Sometimes  when  the  adjust- 
ments of  a  quenched-spark  set 
are  not  correct  it  is  possible  to 
detect  two  wave  lengths,  but  of 
very  small  amplitude,  in  addi- 
tion to  the  single  wave  length 
mentioned  above,  one  of  these 
being  of  shorter  and  the  other  of 
longer  wave  length  than  the  nor- 
mal. The  development  of  these 
two  wave  lengths  is  generally 
due  to  excessive  coupling  so  that 
the  spark  is  not  quenched  at  the  proper  point  but  allows  one  or  two 
transfers  of  the  secondary  energy  back  into  the  primary  during  which 
two  wave  lengths  are  produced.  After  the  spark  is  properly 
quenched,  the  energy  is  retained  in  the  secondary  and  the  normal 
wave  length  of  much  greater  amplitude  is  developed. 

ARRANGEMENTS   AT  THE   RECEIVING   STATION. 

The  electromagnetic  waves  as  they  sweep  across  the  antenna  at 
the  receiving  station  generate  damped  alternating  currents  therein 
of  the  same  frequency  as  those  in  the  transmitting  antenna.  At 
great  distances  the  oscillations  or  currents  are  exceedingly  feeble, 
perhaps  only  a  few  millionths  of  an  ampere,  and  it  requires  correctly 
adjusted  circuits  and  very  sensitive  devices  to  detect  them.  The 
various  types  of  receiving  circuits  will  be  described  next,  and  the 
detectors  later. 


Coup/ing 

\ 
Sec.  \ 


Coupling 


RADIOTELEGRAPH  Y. 


69 


It  is  evident  that  the  strongest  oscillations  will  be  produced  in  the 
receiving  antenna  when  it  has  the  same  frequency  or  wave  length  as 
the  transmitting  antenna.  In  "the  simplest  case  an  an- 
tenna identical  in  construction  with  that  at  the  trans- 
mitting station  can  be  used,  in  which  the  detector  is 
inserted  directly  in  the  antenna,  as  shown  in  figure  58. 
This  circuit  is  sometimes  known  as  the  plain  Marconi 
antenna  for  receiving  and  corresponds  to  the  transmitting 
circuit  of  figure  35.  Owing  to  its  many  disadvantages, 
such  as  trouble  from  static,  interference,  etc.,  this  circuit, 
like  the  plain  transmitting  circuit,  is  not  now  used  in 
practical  radiotelegraphy. 


DIRECTLY    CONNECTED    CIRCUITS. 

A  simple  circuit  for  tuning  the  receiving  antenna  to  the 
same  frequency  or  wave  lengths  as  the  transmitter  is 
shown  in  figure  59,  where  the  adjustments  are  made  by 
using  a  variable  inductance;  thus  the  larger  the  number 
of  turns  in  circuit  the  greater  the  inductance  and  the 
lower  the  frequency  or  the  longer  the  wave  length  of  FIG.  55. 
the  oscillations  to  which  it  is  tuned,  and,  vice  versa,  the  fewer  the 
number  of  turns  the  less  the  inductance  and  the  higher  the  fre- 
quency or  the  shorter  the  wave 
length  of  the  oscillations.  In  this 
case  the  detector  D  is  in  a  branch 
circuit  with  the  condenser  S  and 
the  telephones  T,  which  is  con- 
nected across  a  variable  number  of 
turns  by  means  of  a  sliding  con- 
tact. It  is  seen  that  the  detector 
circuit  is  thus  connected  directly  to 
the  antenna  inductance  coil  and 
hence  is  called  a  directly  connected 
or  directly  coupled  receiving  set, 
thus  corresponding  to  the  direct 
connected  transmitting  sets  of  fig- 
ures 37,  38,  and  39.  This  circuit 
is  of  a  type  similar  to  that  in  the 
double-slide  tuning  coil  sets  for- 
merly used  by  the  Signal  Corps.  In 
FIG.  59.  order  to  be  able  to  tune  the  antenna 

circuit  to  wave  lengths  shorter  than  the  fundamental,  as  is  often 
necessary,  a  series  condenser  must  be  used,  as  shown  in  figure  60. 


Q 


70 


RADIOTELEGRAPH  Y. 


INDUCTIVELY    CONNECTED    CIRCUITS. 

Most  receiving  sets  now  in  use  ar*e  of  the  inductively  connected  or 
inductively  coupled  type,  as  shown  in  figures  61  and  62,  in  which  it 
is  seen  that  the  oscillations  in  the  tuned  antenna  circuit  induce  oscil- 
lations in  a  circuit  coupled  with  it,  thus  corresponding  to  the  induc- 
tively coupled  transmitting  sets  of  figures  36  and  40.  In  this  case  the 
antenna  circuit  is  the  primary  and  its  coil  Lj  is  generally  called  the 
primary  coil  of  the  receiving  transformer.  The  closed  circuit  is  the 
secondary  circuit  and  its  coil  L2  is  the  secondary  of  the  receiving 
transformer.  It  is  to  be  noted  that  these  terms  are  the  reverse  of  those 
used  in  the  transmitting  circuit.  Circuits  of  the  inductively  con- 
nected type  have  advantages  over  those  of  the  directly  connected 

type,  in  that  they  can  generally  be 
rendered  less  liable  to  static  dis- 
turbances and  will  have  sharper 
tuning,  so  that  it  is  more  nearly 
possible  to  cut  out  undesired  sta- 
tions, etc. 

The  closed  or  secondary  circuits 
are  of  two  general  types,  called  the 
untuned  and  tuned,  as  shown  re- 
spectively in  figures  61  and  62. 

In  the  untuned  circuit  there  is  no 
secondary  tuning  condenser,  the 
only  adjustment  being  in  the  num- 
ber of  turns  in  L2,  which  is  gener- 
ally in  steps  of  many  turns.  In  the 
adjustment  of  such  a  set  to  get  sig- 
nals of  maximum  loudness,  the  cir- 
cuits must  be  adjusted  to  resonance, 
and  the  proper  coupling  between 
FlG-60-  them  must  be  used.  The  primary 

circuit  will  be.  sharply  tuned,  but  the  secondary  only  very  broadly 
tuned  if  at  all.  If  a  close  coupling  is  used  between  the  circuits 
the  tuning  of  both  will  be  broad,  and  hence  the  set  will  have  the 
disadvantage  of  being  liable  to  severe  interference.  Under  certain 
conditions,  however,  as  in  searching  for  an  unknown  station,  it  may 
be  of  advantage  to  use  this  coupling  at  first,  and  then  when  the  sta- 
tion has  been  picked  up,  to  loosen  the  coupling  and  to  make  such 
changes  in  both  circuits  as  will  give  the  sharpest  tuning  and  the 
loudest  signals.  In  many  receiver  sets  of  this  type  the  so-called 
untuned  secondary  circuit  is  really  a  broadly  tuned  one  in  which  the 
inductance  of  the  coil  and  its  distributed  capacity  form  the  tuning 
elements.  The  range  of  wave  lengths  to  wilich  each  step  is  thus 


RADIOTELEGRAPHY. 


71 


broadly  tuned  is  generally  marked  for  each  contact,  thus  400  to  600 
meters,  600  to  1,000  meters,  etc. 

In  the  tuned  circuit  there  is  a  variable  tuning  condenser,  as  C2 
in  figure  62,  the  adjustment  of  which  is  necessary  to  secure  the  maxi- 
mum loudness  of  signals.  The  secondary  inductance  is  sometimes 
variable^by'steps  and  in  a  few  cases  by  single  turns.  It  must  be  noted 
that  adjustments  for  any  wave  length  can  be  made  with  different 
combinations  of  inductance  and  capacity.  In  general  it  will  be  found 
that  in  both  the  primary  and  secondary  circuits  there  is  a  best  value 
of  these  combinations  of  inductance  and  capacity  for  any  given  trans- 


FIG.  61. 

mitting  station,  and  that  these  combinations  may  be  different  for 
each  different  station,  and  hence  must  be  found  by  trial.  The  tuning 
of  the  inductively  coupled  receiving  set  requires  a  careful  adjustment 
of  both  circuits  and  of  the  coupling  between  them.  The  three  ad- 
justments are  all  dependent,  one  on  the  other,  so  that  if  the  circuits 
are  adjusted  to  resonance  with  loose  coupling  and  the  coupling  is 
then  increased  and  made  close,  the  circuits  will  be  put  out  of  reso- 
nance and  retuning  of  both  is  necessary.  Similarly,  if  the  circuits  are 
closely  coupled  and  then  each  is  tuned,  it  may  be  found  that  there 
are  two  points  of  resonance  or  two  wave  lengths  in  each  circuit, 


72 


EADIOTELEGKAPHY. 


although  only  a  single  wave  length  is  being  radiated  by  the  trans- 
mitting station.  On  account  of  these  changes  in  wave  length  with 
changes  in  coupling,  it  is  best  to  work  with  as  loose  a  coupling  as 
possible  in  this  type  of  receiver;  also  the  tuning  will  be  sharper 
and  the  interference  will  be  less  under  these  conditions.  There  is 
an  additional  advantage  in  some  cases,  as  the  secondary  circuit  can 
be  calibrated  in  wave  lengths  for  different  settings  of  the  condenser, 
and  hence  the  wave  lengths  of  the  received  signals  measured  at  the 
time  of  reception.  The  best  value  of  the  coupling  will  depend  not 
only  on  the  constants  of  the  circuits,  but  also  upon  the  character  of 


c, 


FIG.  62. 

the  waves  radiated  by  the  transmitter.  The  broader  the  tuning  in 
the  transmitting  station  or  the  larger  the  damping  of  the  waves  radi- 
ated by  it,  the  closer  may  be  the  coupling  between  the  circuits  and  vice 
versa;  the  sharper  the  tuning  in  the  transmitting  station,  or  the 
smaller  the  damping  of  the  radiated  waves,  the  looser  must  be  the 
coupling  between  the  circuits.  In  some  cases  in  actual  practice  it  is 
found  that  when  sustained  or  undamped  waves  are  used,  the  damping 
of  which  is  zero,  the  coupling  between  the  circuits  must  be  made  so 
loose  that  signals  of  the  same  wave  length  from  a  station  using 
highly  damped  waves  may  not  be  heard  at  all. 


RADIOTELEGRAPHY. 


73 


STATIC    AND    INTERFERENCE. 

The  elimination  of  static  disturbances  and  interference  from  other 
stations  is  one  of  the  most  difficult  problems  in  radiotelegraphy.  At 
the  present  time  it  is  doubtful  if  there  is  a  complete  solution  of  both 
troubles.  The  elimination  of  static  is  dependent  largely  on  the 
design  of  the  apparatus  at  the  receiving  station,  whereas  that  of 
interference  is  dependent  on  both  the  transmitting  and  the  receiving 
apparatus.  A  few  of  the  simpler  means  of  reducing  static  may  be 
mentioned  as  follows :  In  some  cases  static  can  be  cut  down  by  con- 
necting a  very  high  resistance,  as  10,000  ohms  or  more,  between  the 
antenna  and  ground,  thus  giving  a  shunt  path  to  earth  for  the  static- 
In  many  cases  a  very  loose  coupling  between  the  receiver  circuits 
may  reduce  the  static  more  than  the  desired  signals,  which  although 


Wave  Length 

FIG.  63. 

much  weakened  can  still  be  read.  When  the  transmitted  signals  are 
of  high  pitch  they  can  be  read  through  moderate  static  much  easier 
than  those  of  low  pitch,  as  mentioned  on  page  76.  If  the  dia- 
phragms of  the  receiving  telephones  are  tuned  to  the  pitch  of  the 
transmitted  signals  the  static  can  be  still  further  eliminated.  There 
are  many  types  of  circuits  which  have  been  suggested  as  useful  in 
reducing  static,  which  although  effective  in  stations  with  small  an- 
tennas are  often  of  little  use  with  the  large  antennas  which  must  be 
used  in  powerful  transmitting  stations.  This  fact  has  sometimes  led 
to  the  installation  of  two  antennas  at  a  station,  a  large  one  only  for 
transmitting  and  a  small  one  of  two  or  three  wires  for  receiving.  It 
is  often  possible  then  to  get  messages  on  the  small  antenna  that  can 
not  be  copied  on  the  large  one. 


74 


RADIOTELEGRAPHY. 


The  elimination  of  interference  is  dependent  on  both  the  trans- 
mitter and  receiver  design.  The  more  nearly  that  the  transmittting 
oscillations  are  undamped;  that  is,  the  more  sharply  that  the  ra- 
diated energy  is  confined  to  single  wave  lengths;  and  at  the  same 
time  the  lower  the  resistance  of  the  receiver  circuits  and  the  more 
sensitive  the  detector,  the  more  certainly  is  it  possible  to  prevent 
interference.  Thus  if  two  stations  have  transmitters  whose  radiated 
wave  lengths,  as  tested  by  a  wave  meter,  are  as  shown  by  a  in  figure 
63,  and  have  receivers  whose  circuits  permit  of  reception  of  wave 
lengths  as  shown  by  a  in  figure  64,  it  is  evident  that  they  can  work 
together  without  causing  interference  at  other  stations  and  without 
being  subject  to  interference  except  at  lengths  very  near  their  own. 
On  the  other  hand,  if  two  stations  radiate  waves  as  shown  by  b  in 
figure  63,  and  receive  wave  lengths  as  shown  by  6  in  figure  64,  it  is 


Wave  Length 

FIG.  64. 

evident  that  they  will  cause  interference  at  other  stations  on  account 
of  the  broad  tuning  of  the  transmitters  and  will  be  subject  to  inter- 
ference on  account  of  the  -broad  tuning  of  the  receiving  circuits. 

There  are  many  types  of  circuits  which  have  been  found  useful  in 
helping  to  prevent  interference,  one  of  the  simplest  of  which  is  the 
loosely  coupled  inductive  receiving  set  as  shown  in  figure  62.  When 
these  circuits  are  of  low  resistance,  the  inductance  and  capacity  of 
each  circuit  variable  so  as  to  secure  the  best  combination  of  the 
two,  and  the  coupling  as  loose  as  the  signals  permit,  such  a  set 
can  be  used  to  receive  signals  at  any  one  wave  length  from  one 
station  and  to  exclude  signals  of  slightly  different  wave  lengths 
from  other  stations.  This  property  of  the  reception  of  signals  of 
one  wave  length  and  the  exclusion  of  those  of  other  wave  lengths 
is  called  selectivity  and  such  a  receiver  is  said  to  be  selective.  In 
figure  65  is  shown  a  receiving  set  which  is  provided  with  an  addi- 


EADIOTELEGEAPHY. 


75 


tional  circuit  of  coil  L\  and  condenser  C\  connected  between  the 
antenna  and  ground,  which  with  the  antenna  is  tuned  to  the  wave 


FIG.  65. 


length  of  the  interfering  station  and  thus  furnishes  a  tuned  shunt 
path  to  ground  for  the  undesired  signals.    This  is  sometimes  called 


FIG.  66. 


an  interference  minimizer  circuit.     The  connection  of  this  circuit  to 
the  antenna  will  slightly  change  the  tuning  of  the  primary  circuit 


76  RADIOTELEGRAPH  Y. 

so  that  both  have  to  be  adjusted  together,  one  to  decrease  the  un- 
desired  signals  to  a  minimum  and  the  other  to  increase  the  desired 
signals  to  a  maximum.  In  order  to  prevent  the  grounding  of  the 
desired  wave  lengths  by  the  shunt  circuit  at  times  when  it  is  not 
needed,  the  circuit  should  be  opened  by  a  switch  as  shown  in  the 
figure.  In  figure  66  is  shown  a  somewhat  similar  type  of  circuit 
for  reducing  interference  by  absorbing  the  undesired  wave  lengths, 
the  circuit  being  coupled  with  the  antenna  circuit  as  needed  and 
tuned  to  the  interference  which  will  be  reduced  thereby.  In  order 
to  prevent  the  desired  wave  lengths  from  being  absorbed  by  the  cir- 
cuit when  it  is  not  needed,  the  circuit  should  be  opened  by  a  switch 
as  in  the  case  of  the  other  circuit. 

DETECTORS. 

The  form  of  detector  first  used  in  radiotelegraphy  was  the  coherer, 
which  by  means  of  a  decoherer  or  electric  vibrator,  like  an  electric 
bell,  continually  restored  the  sensibility  of  the  coherer  to  currents 
produced  in  the  antenna,  and  permitted  the  signals  to  be  received 
on  a  relay  and  sounder.  The  coherer  is  not  now  used  in  practical 
work,  having  been  replaced  by  other  more  sensitive  and  satisfactory 
types  of  detectors. 

An  important  improvement  in  sensibility  and  certainty  of  opera- 
tion was  made  by  the  introduction  of  the  telephone  receiver  as  the 
receiving  instrument  instead  of  the  sounder,  the  dots  and  dashes  be- 
ing received  as  short  and  long  buzzing  sounds  of  the  same  audible 
frequency  or  note  as  that  at  the  transmitting  station.  Experiments 
have  shown  that  the  ear  is  more  sensitive  to  notes  of  a  JiigJi  pitch,  as 
several  hundred  or  a  thousand  vibrations  per  second,  the  latter  being 
given  by  a  500-cycle  alternator,  than  to  notes  of  a  low  pitch,  as  120 
vibrations  per  second,  as  given  by  a  60-cycle  alternator.  It  has  also 
been  found  easier  to  read  a  note  of  high  pitch  than  one  of  low  pitch 
in  static  or  other  irregular  disturbances.  These  are  two  advantages 
of  the  high-spark  frequency  or  high- wave  train  frequency  at  the 
receiving  station,  the  corresponding  advantages  at  the  transmitting 
station  having  already  been  mentioned. 

The  high-frequency  currents  in  the  receiving  antenna  have  a  fre- 
quency of  from  say  50,000  to  over  1,000,000  per  second,  but  as  the 
telephone  diaphragm  can  not  vibrate  at  this  great  frequency,  the 
telephone  receiver  can  not  be  used  directly  as  a  radio  receiver.  Even 
if  the  diaphragm  could  vibrate  at  this  frequency  we  would  be  unable 
to  detect  any  sounds,  as  the  human  ear  does  not  respond  to  more  than 
about  20,000  vibrations  per  second.  It  is  evident,  then,  that  the  tele- 
phone receiver  itself  can  not  make  the  signals  audible,  but  that  it 
must  be  used  in  connection  with  some  of  the  detectors  described  below. 


RADIOTELEGRAPHY. 


77 


A  number  of  forms  of  detectors  have  been  invented,  most  of  which 
rectify  the  high-frequency  currents — that  is,  change  them  from  alter- 
nating to  direct  currents  by  some  kind  of  valve  action — and  thus  ren- 
der them  capable  of  operating  the  telephone  at  an  audible  frequency. 
In  figure  67  the  upper  curve  shows  several  damped  wave  trains,  the 
middle  curve  shows  them  as  theoretically  rectified  by  the  detector  so 


r\ 


A 


LAA 


A 


A 


A 


FIG.  67. 


that  the  current  is  allowed  to  pass  only  in  one  direction,  and  the  lower 
curve  the  actual  current  through  the  telephone,  where  each  wave 
train'is  practically  the  equivalent  of  a  direct  current  lasting  a  small 
fraction  of  a  second,  or  a  pulsating  current,  as  it  is  often  called. 
Thus,  in  the  case  of  a  spark  frequency  of  1,000  per  second,  the  tele- 


FlG.  68. 


phone  is  operated  as  though  by  a  direct  current  interrupted  1,000 
times  per  second. 

The  electrolytic  detector,  as  shown  in  figure  68,  consists  of  a  small 
glass  cup,  A,  containing  dilute  nitric  acid,  four  parts  of  water  to  one 


7  8  RADIOTELEGRAPH  V . 

of  concentrated  acid,  with  a  small  piece  of  platinum,  B,  projecting 
through  the  bottom  a  platinum  wire,  W,  of  very  small  diameter 
carried  by  a  screw  may  be  lowered  until  it  just  enters  the  liquid. 
Sometimes  this  wire  is  sealed  in  a  glass  tube  with  its  end  just 
flush  with  the  glass,  the  diameter  of  the  wire  being  about  TOTHT 
(0.001)  inch.  In  the  bare-wire  type  the  platinum  forms  the  core  of 
a  silver  platinum  wire,  the  silver  of  which  must  first  be  dissolved  off 
by  the  action  of  the  acid  and  the  battery,  leaving  the  platinum  with 
a  diameter  of  perhaps  I0ooo  (0.0002)  inch.  The  battery,  F,  of  two 
dry  cells  is  connected  to  the  high-resistance  wire,  D,  of  200  ohms 
or  more  and  an  adjustable  connection,  E,  goes  to  the  detector.  This 
connection  must  be  taken  from  the  positive  or  carbon  terminal  of 
the  battery  so  as  to  make  the  point  positive  and  the  plate  negative. 
By  sliding  this  contact  along  the  voltage  between  B  and  C  on  the  de- 
tector may  be  regulated.  This  device  is  called  a  potentiometer.  At  a 
certain  adjustment,  which  may  be  detected  by  a  frying  sound  in  the 
telephone  receiver,  T,  the  current  begins  to  flow  through  the  acid 
from  the  point  to  the  plate,  evolving  gas  at  the  points  B  and  C,  thus 
polarizing  the  detector,  as  it  is  said.  Just  before  this  point  is  reached 
there  is  a  balance  where  the  polarization  in  the  detector  blocks  the 
battery  current.  When  this  state  of  things  exists,  the  production  of 
high  frequency  oscillations  in  the  detector  circuit,  induced  therein 
from  the  antenna  circuit,  disturbs  the  polarization  in  the  detector, 
and  as  a  result  rectified  or  pulsating  currents  from  the  battery  through 
the  detector  in  the  direction  of  point  to  plate  and  through  the  tele- 
phones give  audible  signals  in  the  telephone  of  the  same  frequency 
as  the  spark  frequency  of  the  transmitter. 

Other  kinds  of  detectors,  sometimes  called  crystal  or  contact  de- 
tectors, consist  of  various  substances  in  light  contact,  such  as  steel- 
carborundum,  steel-silicon,  etc.;  metallic  contact  on  pyrite,  galena, 
etc. ;  zincite-chalcopyrite,  silicon-arsenic,  silicon-antimony,  etc.  These 
have  all  been  patented,  and  some  of  them  have  received  trade  names, 
such  as  u  perikon"  for  zincite-chalcopyrite,  "pyron"  for  metallic 
contact  on  pyrite,  etc.  In  the  case  of  the  perikon,  silicon-arsenic, 
silicon-antimony,  etc.,  the  materials  are  embedded  in  flat  buttons  of 
fusible  alloy  or  solder  on  an  adjustable  holder  and  held  in  light  con- 
tact by  a  spring;  in  the  steel-silicon,  pyrite,  galena,  etc.,  contact  is 
made  by  a  light  wire  spring  on  a  universal  jointed  holder. 

Some  of  these  detectors  are  sensitive  to  the  high-frequency  oscil- 
lations without  the  application  of  an  external  electromotive  force,  as 
the  steel-silicon,  galena,  etc.,  and  the  simplest  circuit  in  this  case  is 
shown  in  figure  69,  where  D  is  the  detector,  T  the  telephones,  and  S 
a  fixed  condenser  of  about  0.003-microfarad  capacity.  Other  de- 
tectors are  more  sensitive  when  a  small  electromotive  force,  as 
from  a  potentiometer,  is  applied  to  them  as  the  perikon,  pyron,  etc., 
and  in  this  case  the  circuit  is  shown  in  figure  70,  where  D  is  the  de- 


EADIOTELEGEAPHY. 


79 


tector,  T  the  telephones,  S  the  condenser,  generally  fixed,  but  some- 
times variable  by  steps. 
Another  type  of  detector  called  the  "audion,"  shown  in  figure  71, 


FIG.  69. 


consists  essentially  of  a  partially  exhausted  bulb  in  which  has  been 
sealed  a  metallic  filament,  F,  a  small  plate,  P,  and  a  grid  of  wires, 
G  each  insulated  from  the  others.  The  filament  is  heated  to  incan- 


FlG.  70. 


descence  by  a  storage  battery,  A,  often  called  the  ""A  battery/' 
of  about  6  volts,  the  current  from  which  is  regulated  by  means 
of  a  small  rheostat,  R.  The  plate  P,  entirely  insulated  within 


80 


RADIOTELEGRAPH  Y. 


the  bulb,  is  connected  to  one  terminal  of  the  telephones,  T,  the  other 
one  of  which  is  connected  to  a  battery  of  small  dry  cells,  B,  often 
called  the  UB  battery,"  of  30  to  50  volts,  the  number  of  which  in 
circuit,  and  hence  the  voltage  is  controlled  by  a  switch.  The  ter- 
minals of  the  detector  circuit  are  connected,  one  to  the  base  of 
the  filament  and  the  other  to  the  insulated  wire  grid,  G,  through 
a  small  stopping  condenser,  S.  The  action  of  the  audion  seems  to 
be  that  of  a  relay  and  its  operation  is  as  follows:  Under  the  influ- 
ence of  the  hot  filament  the  molecules  of  gas  remaining  in  the  bulb 
acquire  the  property  of  conducting  a  small  current  on  the  applica- 
tion of  30  to  50  volts  in  the  direction  of  filament  to  plate,  but 
not  in  the  reverse  direction,  and  if  the  telephone  is  connected  in 


A  'Battery 


FIG.  71. 

this  circuit  as  shown  a  small,  steady  current  will  flow  through  it. 
On  the  arrival  of  the  high-frequency  oscillations  at  the  grid  and  the 
filament  it  is  probable  that  they  can  flow  only  in  one  direction,  and 
during  their  passage  over  part  of  the  path  of  the  telephone  current 
they  change  its  resistance,  and  hence  the  current  in  the  telephones, 
and  thus  make  audible  signals.  For  reasons  previously  given,  the 
pitch  of  the  note  in  the  telephones  is  the  same  as  that  of  the  spark 
frequency  at  the  transmitting  station. 

A  sensitive  detector  of  a  somewhat  novel  type  is  now  coming  into 
use  called  the  ticker,  consisting  essentially  of  fine  steel  or  other  wire 
resting  with  light  contact  in  a  groove  on  a  rotating  disk  of  brass 
or  other  suitable  material.  This  detector  can  be  used  instead  of  D 


RADIOTELEGRAPH  Y.  81 

in  the  circuit  shown  in  figure  69  in  which  the  condenser  S  should  now 
be  about  0.01  mf.  and  the  telephones  of  low  resistance. 

TELEPHONES. 

The  telephone  receivers  used  in  detector  circuits  are  wound  to  a 
"high  resistance,  as  1,000  ohms  or  more  for  each  one  of  a  pair.  The 
reason  for  this  is  as  follows:  The  movements  of  the  telephone  dia- 
phragm are  caused  by  the  attraction  of  the  telephone  magnet,  which 
increases  as  the  product  of  the  current  in  the  telephone  and  the 
number  of  turns  in  the  windings.  As  the  current  from  the  detector 
is  very  small,  it  is  evident  that  a  large  number  of  turns  must  be  used 
to  secure  the  necessary  attraction,  and  hence  the  telephone  becomes 
one  of  high  resistance. 

Every  telephone  diaphragm  has  a  certain  natural  period  of  me- 
chanical vibration  or  pitch.  When  the  incoming  signals  are  of  the 
same  pitch — that  is,  they  are  in  resonance  with  the  period  of  the 
diaphragm — these  signals  will  be  heard  louder  than  others  from 
transmitters  of  the  same  power  but  of  different  pitch.  In  some  cases 
the  natural  pitch  of  a  diaphragm  may  coincide  with  that  of  the  sig- 
nals, and  thus  the  telephone  will  be  found  to  be  very  sensitive.  The 
pitch  of  the  diaphragm  can,  however,  be  changed  by  changing  the 
distance  between  it  and  the  magnet,  and  some  types  of  telephones 
are  supplied  with  adjustable  pole  pieces.  By  this  means  it  is  possi- 
ble to  tune  the  telephone  itself  to  resonance  with  the  spark  fre- 
quency of  the  transmitter  and  often  increase  the  loudness  of  the 
signals. 

The  fixed  condenser  is  shunted  across  the  telephone  terminals 
in  order  to  provide  a  complete  circuit  for  the  oscillations  between 
the  condenser  terminals  without  having  to  flow  through  the  tele- 
phones, the  high  inductance  of  which  in  circuit  would  tend  to  choke 
back  the  oscillations  and  so  possibly  prevent  their  detection.  It  is 
evident  that  a  very  large  condenser  can  not  be  used,  as  it  would 
serve  as  such  a  low-impedance  shunt  for  the  pulsating  currents  from 
the  detector  that  no  current  would  flow  through  the  telephone,  and 
on  the  other  hand  a  very  small  condenser  can  not  be  used,  as  it  would 
not  allow  the  oscillations  to  flow  through  it.  The  best  value  must  then 
be  determined  by  trial  and  it  is  found  in  practice  to  vary  slightly 
with  the  spark  or  wave  train  frequency.  With  the  high-resistance 
telephones  in  general  use  the  capacity  of  the  condenser  is  about  0.003 
to  0.0035  mf.  for  low  frequencies,  as  60  cycles,  and  about  0.002  to  0.003 
mf .  for  high  frequencies,  as  500  cycles.  In  some  cases  this  condenser 
is  variable  by  steps  so  as  to  be  able  to  adjust  to  different  spark  fre- 
quencies or  to  group  tuning,  as  it  is  sometimes  called.  By  the  use 
of  such  a  variable  condenser  and  of  a  telephone  with  adjustable  pole 
92448—15 6 


82 


RADIOTELEGRAPH!'. 


pieces  it  is  often  possible  to  increase  the  loudness  of  signals  and  the 
selectivity  of  the  circuits  without  making  changes  in  the  tuning. 

In  some  types  of  circuits  the  fixed  condenser  serves  another  pur- 
pose, as  shown  in  figure  68,  where  it  prevents  the  short  circuiting  of 
the  battery  by  the  coil,  in  which  case  it  is  often  called  the  stopping 
or  blocking  condenser. 

CALIBRATING     WAVE     LENGTHS     OF     RECEIVING     CIRCUITS     BY 
MEANS   OF   THE    WAVE    METER. 

In  the  previous  illustrations  of  the  wave  meter  it  was  used  to 
receive  oscillations  and  to  measure  their  wave  lengths.  It  may, 
however,  be  used  to  send  out  oscillations  of  known  wave  lengths  of 
comparatively  feeble  intensity,  in  which  c"ase  several  types  of  circuits 
may  be  used  to  excite  the  meter,  as  by  means  of  a  buzzer  shown  in 
figure  72,  where  A  is  a  battery  of  not  more  than  two  dry  cells,  B  is 
the  buzzer,  and  L  C  is  the  meter.  This  circuit  is  sometimes  known 
as  the  buzzer  method  of  excitation  of  the  wave  meter  which  thereby 
becomes  a  source  of  feebly  damped  oscillations;  thus,  if  a  circuit  is 

B 


It 


brought  near  the  coil  L  and  loosely  coupled  with  it  the  meter  will 
induce  in  the  circuit  oscillations  of  the  wave  length  or  frequency 
corresponding  to  the  setting  of  the  wave  meter  condenser.  The  cir- 
cuits of  a  station  receiver  connected  to  the  station  antenna  may  be 
calibrated  by  this  method.  The  action  of  the  buzzer  circuit  seems 
to  be  that  at  each  spark  at  the  buzzer  contacts  the  meter  condenser 
is  charged  and  then  discharged  through  the  inductance  and  thus 
sets  up  oscillations,  independently  of  the  charging  circuit  in  a  man- 
ner similar  to  that  of  the  closed  circuit  as  charged  by  the  secondary 
of  the  A.  C.  transformer. 

This  circuit  may  be  used  in  making  many  useful  measurements  and 
tests  in  radio  work,  such  as  inductance,  capacity,  sensitiveness  of  tele- 
phones, detector,  etc. 

RADIO    APPARATUS   IN   USE    IN  THE    SIGNAL   CORPS. 

The  Signal  Corps  has  installed  10  radio  stations  in  Alaska,  varying 
in  size  from  1  kilowatt  at  Petersburg,  Wrangell,  and  Kotlik  to  10 
kilowatts  at  Fort  Gibbon,  Nulato,  and  Nome.  Stations  of  from  3  to 


KADIOTELEGKAPHY.  83 

5  kilowatts  have  been  installed  at  St.  Michael,  Circle,  Fairbanks,  and 
Fort  Egbert. 

In  the  Philippines  stations  have  been  installed  at  Manila,  Fort 
McKinley,  and  Fort  Wint,  and  a  station  of  10  kilowatts  at  Corregidor. 

In  the  United  States  a  1  or  2  kilowatt  set  has  been  installed  in 
several  of  the  Coast  Artillery  districts;  3-kilowatt  sets  at  Fort  Wood, 
Fort  Omaha,  and  Fort  Riley;  and  a  10-kilowatt  set  to  be  installed  at 
Fort  Leavenworth.  Sets  of  from  1  to  5  kilowatts  have  been  in- 
stalled on  14  transports  and  3  cable  ships,  and  sets  of  from  one-eighth 
to  2  kilowatts  on  the  harbor  boats  assigned  to  Coast  Artillery  districts 
that  have  a  shore  station. 

All  the  Alaska  and  the  Philippine  stations  except  Corregidor 
have  their  generators  driven  by  gasoline  engines.  The  generators 
in  the  Artillery  districts  and  on  the  harbor  boats  are  nearly  all  driven 
by  motors  from  local  electric  power.  The  Fort  Wood  station  may 
be  operated  either  from  a  gasoline  engine  or  the  local  electric-light 
plant.  The  Fort  Omaha,  Fort  Riley,  and  Fort  Leavenworth  sets  are 
operated  directly  from  city  power. 

Two  types  of  portable  field  sets  have  been  issued  by  the  Signal 
Corps.  The  smaller  size  is  furnished  to  the  Organized  Militia  as 
well  as  to  the  field  companies,  and  is  described  on  pages  102  to  119. 
The  range  of  these  sets  under  normal  conditions  is  about  25  miles 
over  land,  but  much  greater  over  water.  Thus  one  of  the  one-eighth 
kilowatt  sets,  with  a  100-foot  mast,  at  Habana  has  worked  with  the 
naval  station  at  Key  West,  a  distance  of  about  110  miles. 

The  larger  size  of  field  sets  is  described  on  pages  91  to  102.  It  is 
of  2-kilowatts  output  and  is  carried  on  a  two  chest  pintle  wagon,  one 
chest  with  the  engine  and  generator  and  the  other  with  the  trans- 
mitting and  the  receiving  apparatus.  The  range  of  these  sets  varies 
from  75  to  800  miles,  depending  on  favorable  weather  conditions, 
time  of  day  or  night,  character  of  the  land  between  the  sets,  etc. 

FORT    SAM  HOUSTON    STATION    SET. 

The  following  description  of  the  Fort  Sam  Houston  station  is 
given  as  an  illustration  of  the  type  of  the  10-kilowatt  sets  installed 
by  the  Signal  Corps  in  Alaska  and  elsewhere  in  the  United  States. 

Towers. — These  are  of  structural  steel,  about  200  feet  high,  28  feet 
square  at  base,  and  4  feet-  square  at  top.  The  towers  are  supported 
on  concrete  piers,  each  leg  resting  on  a  cribwork  of  timbers  12  inches 
square,  painted  with  insulating  compound  for  preservation  and  in- 
sulation. Timbers  are  bolted  to  the  piers  and  to  each  other,  the  bolts 
from  the  towers  not  extending  down  into  the  concrete.  The  towers 
are  about  350  feet  apart. 

Antenna. — The  antenna  is  of  the  T  type,  the  flat  top  part  of 
which  is  composed  of  four  wires,  each  475  feet  long  and  8  feet  apart, 


RADIOTELEGRAPHY. 


the  wires  being  carried  beyond  the  towers  to  backstays.  Both 
ends  of  these  wires  are  insulated  with  18-inch  electrose  insulators- 
The  details  of  the  insulation,  spars,  bridles,  etc.,  are  shown  in  fig- 
ure 73.  The  vertical  wires,  reaching  from  the  center  of  the  flat  top 


to  the  station,  are  each  180  feet  long,  separated  8  feet,  and  at  the 
bottom  are  joined  together  and  carried  as  a  single  wire  for  about 
10  feet  into  the  station  through  a  porcelain  wall  insulator. 


RADIOTELEGRAPH  Y.  85 

Counterpoise  and  ground. — Connections  are  made  to  the  water- 
pipe  system  as  a  ground,  but  the  most  dependence  is  placed  on  a 
counterpoise,  described  on  page  54,  which  covers  about  half  an  acre 
of  land. 

Power  equipment. — The  alternator  is  belted  to  a  single-phase,  60 
cycle,  20  horsepower  induction  motor  driven  by  electric  power  fur- 
nished from  San  Antonio.  The  motor  can  be  automatically  started 
by  closing  a  switch  on  the  operator's  table.  In  places  where  such 
power  is  not  available,  as  in  Alaska,  a  Fairbanks  &  Morse  20-horse- 
power  gasoline  engine  is  generally  used.  The  motor  speed  is  1,750 
R.  P.  M.,  the  diameter  of  its  driving  pulley  is  12  in.,  the  diameter 
of  the  driven  pulley  on  the  generator  is  14  J  in.,  thus  giving  the 
normal  generator  speed  of  1,500  R.  P.  M.  This  machine  is  of  the 
inductor  type  separately  excited  by  a  1.5  kilowatt,  D.  C.  exciter  on 
the  same  shaft  as  the  A.  C.  armature,  and  delivers  the  power  of  10 
kilowatts,  at  a  frequency  of  500  cycles  and  at  220  volts. 

Switchboard. — The  switchboard  is  mounted  close  to  the  operating 
table  and  contains  the  500-cycle  frequency  meter,  A.  C.  ammeter  and 
voltmeter,  the  exciter,  D.  C.  ammeter  and  voltmeter,  and  generator 
field  rheostat  for  the  adjustment  of  the  alternator  voltage.  The  500- 
cycle  wattmeter  and  the  antenna  hot-wire  ammeter  are  mounted  else- 
where. 

Transformer. — The  transformer  is  of  the  open  magnetic  circuit 
type  with  dry  insulation,  with  a  reactance  in  its  primary  circuit  for 
the  proper  adjustment  of  these  circuits,  as  mentioned  on  page  24. 

Key. — The  key  is  of  the  relay  type,  controlled  by  an  ordinary  Morse 
key,  which  uses  the  direct  current  from  the  exciter  to  operate  the 
relay.  The  Morse-key  contacts  are  shunted  by  a  condenser  to  cut 
down  the  sparking. 

Condenser. — The  closed-circuit  condenser  consists  of  5  Leyden  jars, 
covered  with  foil,  each  of  a  capacity  of  10,000  cm.  or  0.0111  mf. 

Inductance. — The  closed  circuit  inductance  is  in  the  form  of  a  helix 
wound  with  flat  strip  and  adjustable  only  by  steps  for  certain  pre- 
determined wave  lengths,  contact  being  made  on  the  step  correspond- 
ing to  the  desired  wave  length  and  the  secondary  or  open  circuit 
tuned  to  resonance  with  the  closed  circuit. 

Spark  gap. — The  gap  is  of  the  quenched  type  with  plates  of  copper 
but  with  a  heavy  plate  of  silver  for  the  sparking  surface  as  men- 
tioned on  page  40.  The  separators  are  of  mica.  The  gap  is  cooled  by 
a  blower  driven  by  an  electric  motor  taking  power  from  the  direct 
current  exciter. 

Open  or  radiating  circuit. — As  this  set  is  of  the  directly  connected 
type,  the  closed  circuit  inductance  is  included  in  the  open  circuit. 
The  coupling  is  made  loose  by  the  use  of  antenna  loading  inductance, 
variable  by  steps  for  approximate  resonance,  and  an  antenna  vario- 
meter for  fine  adjustment  between  these  steps  as  described  on  page  35. 


86  KADIOTELEGRAPHY. 

Receiving  set. — Two  sets  have  been  provided,  one  manufactured  by 
the  Telefunken  Co.,  and  the  other  by  the  Wireless  Specialty  Ap- 
paratus Co.,  both  being  of  the  inductively  coupled  type.  In  the  Tele- 
funken receiver  two  primary  coils  are  furnished  so  as  to  secure  a  wide 
range  of  wave  lengths,  and  in  addition  a  primary  condenser  that  can 
be  connected  by  a  switch  either  in  series  with  the  coil  for  short  wave 
lengths  or  in  parallel  for  long  ones.  Similarly  three  secondary  coils 
are  furnished,  one  when  no  secondary  condenser  is  used  and  the  cir- 
cuit is  only  broadly  tuned  and  the  other  two  to  be  used  with  the  sec- 
ondary condenser  when  the  circuit  is  sharply  tuned.  The  detector 
with  the  telephone  and  the  fixed  condenser  is  not  permanently  con- 
nected across  the  terminals  of  the  secondary  condenser  or  coil  as  in 
many  circuits  but  across  a  variable  number  of  turns  in  the  coil. 

The  circuits  in  the  receiver  of  the  Wireless  Specialty  Apparatus 
Co.,  known  as  the  I— P-76  set,  are  similar  to  those  in  the  other^ 
except  that  the  primary  circuit  has  no  condenser  and  hence  can  not 
be  tuned  to  wave  lengths  shorter  than  the  fundamental  wave  length 
of  the  antenna  unless  an  extra  condenser  is  provided.  For  very 
long  wave  lengths  a  loading  inductance,  normally  not  connected  in 
circuit,  can  be  inserted  and  varied  until  resonance  is  obtained.  The 
secondary  circuit  has  a  variable  coil  and  condenser,  across  the 
terminals  of  which  the  detector,  etc.,  is  connected.  As  the  perikoii 
detector  furnished  with  this  receiver  is  more  sensitive  when  a  small 
electromotive  force  is  applied  to  it,  a  potentiometer  is  included  as 
part  of  the  set.  The  telephones  are  of  the  adjustable  pole-piece  type 
as  mentioned  on  page  80. 

COAST   ARTILLERY    STATION   SET. 

The  following  directions  and  instructions  should  be  used  in  the 
installation  and  operation  of  the  1 -kilo watt  Marconi  500-cycle  sets 
supplied  by  the  Signal  Corps  for  use  in  the  Coast  Artillery  stations. 

Installation. — Install  the  motor-generator  in  a  level  position, 
securely  mounted  on  a  solid  foundation,  preferably  of  concrete,  fill 
the  bearings  with  oil,  and  take  care  that  the  oil  rings  are  working 
properly.  Connect  the  apparatus  as  shown  in  figures  74  and  75, 
locating  the  quenched  gap,  oscillation  transformer,  antenna  induc- 
tance, and  switchboard  so  as  to  be  easily  reached  by  the  operator 
at  the  key.  Locate  the  antenna  ammeter  where  it  can  be  easily  seen 
from  the  operator's  seat.  Ground  the  middle  points  of  the  carbon- 
rod  protective  devices  on  some  ground  other  than  the  one  used  for 
the  antenna  circuit.  In  the  case  of  A.  C.  motor-driven  sets,  one- 
half  microfarad  condensers  should  be  used  as  protective  devices  in 
addition  to  the  carbon  rods. 


EADIOTELEGRAPHY. 


87 


Hii- 


88 


EADIOTELEGRAPHY. 


Operation. — The  generator  may  be  driven  either  by  a  D.  C.  or  an 
A.  C.  motor.  In  the  case  of  the  A.  C.  motor  set,  the  machine  starts 
as  a  repulsion  motor,  with  the  armature  short-circuited  through  car- 
bon brushes  on  the  commutator,  and  when  nearly  up  to  full  speed 
the  brushes  are  automatically  lifted  from  the  commutator,  which  is 
short-circuited  at  the  same  time.  This  change  of  connections  con- 


A.  C. 


A.  C. 


A.C. 


Vo/tmeter         Wattmeter          Ammeter 


\J^ - '     '  V-o    Oy  \7>   *>-Z 


FIG.  75. 

verts  the  motor  into  an  induction  motor.  In  motors  of  this  small 
size,  start  the  machine  by  closing  the  main  A.  C.  switch.  No  means 
is  provided  for  the  regulation  of  speed.  In  the  case  of  the  D.  C. 
motor  set,  start  the  machine  by  closing  the  switch  of  the  automatic 
starter  and  adjust  the  speed  by  means  of  the  motor-field  rheostat 
until  the  frequency  meter  reads  500  cycles.  Connect  into  circuit  8 


RADIOTELEGRAPHY.  89 

gaps  of  the  quenched  gap.  Close  the  switch  to  the  generator  fields 
and  adjust  the  generator  voltage  by  means  of  the  generator  field  rheo- 
stat until  the  A.  C.  voltmeter  reads  about  200  volts.  Make  certain 
that  the  spark-gap  blower  is  running,  which  should  have  been  started 
when  the  generator  field  switch  was  closed.  Set  the  switch  of  the 
primary  of  the  oscillation  transformer  on  the  desired  wave  length. 
CAUTION:  Never  move  the  primary  switch  which  controls  the  wave 
length  when  the  key  is  closed.  Pull  out  the  handle  of  the  secondary 
of  the  oscillation  transformer  3  inches  or  more.  Then  close  the  gen- 
erator armature  switch  and  press  the  key.  Rotate  the  handle  of  the 
secondary  of  the  oscillation  transformer  until  the  antenna  ammeter 
shows  a  maximum  reading.  NOTE:  It  is  intended  that  the  Jiandles 
of  the  oscillation  transformer  and  antenna  inductance  can  be  turned 
when  the  key  is  closed  without  danger  of  shock.  If  no  maximum  is 
found  and  if  the  reading  increases  as  the  number  of  turns  in  the 
secondary  increases,  connect  in  some  of  the  turns  in  the  antenna 
loading  coil.  If  no  maximum  is  found  and  if  the  reading  increases 
as  the  number  of  turns  in  the  secondary  decreases,  set  the  switch  of 
the  primary  of  the  oscillation  transformer  on  a  shorter  wave  length. 
Rotate  the  handle  of  the  antenna  inductance  until  a  maximum  is 
found.  In  some  cases  it  may  be  found  that  a  maximum  can  be  found 
without  using  the  antenna  inductance  at  all.  Next  adjust  the  cou- 
pling by  pushing  the  handle  of  the  secondary  in  until  the  highest 
possible  reading  in  the  antenna  ammeter  is  obtained.  It  may  be 
necessary  to  make  slight  changes  in  the  number  of  turns  in  the 
secondary  simultaneously  with  this  adjustment,  but  the  amount  of 
this  change  should  be  not  more  than  one-eighth  to  one-fourth  of 
a  turn.  See  that  the  frequency  meter  reads  500  cycles  when  the  key 
is  closed  and  adjust  the  note  of  the  transmitter  until  it  is  a  clear 
high  whistle  or  note  characteristic  of  this  frequency.  This  note  can 
be  heard  in  the  telephones  of  the  receiving  set  by  leaving  it  connected 
to  the  ground  but  disconnected  from  the  antenna  and  adjusting  the 
detector  until  the  note  is  heard.  If  the  generator  voltage  is  too  low, 
the  note  will  be  clear,  but  of  low  pitch;  if  too  high,  the  note  will  be 
rough  or  hissing.  If  a  clear  note  is  not  obtained  by  the  adjustment 
of  the  generator  voltage,  make  slight  changes  in  coupling  and  pos- 
sibly in  the  number  of  turns  in  the  primary  and  secondary  of  the 
oscillation  transformer  until  the  desired  purity  of  note  is  obtained, 
but  these  changes  should  not  appreciably  reduce  the  antenna  ammeter 
reading.  If  after  these  adjustments  have  been  made,  the  wattmeter 
does  not  read  the  full  1  kilowatt,  open  the  generator  armature  and 
field  switches  to  avoid  the  danger  of  a  shock  and  connect  in  two  or 
three  more  gaps  to  give-  the  necessary  increase  in  power.  Close  both 
switches  and  increase  the  generator  voltage  until  the  note  is  again 
clear  and  of  the  proper  pitch.  CAUTION:  Never  touch  any  circuit 


90  11ADIOTELEGRAPHY. 

which  may  be  alive  without  first  opening  the  generator  field  or  arma- 
ture switches,  preferably  both;  note  that  opening  the  key  does  not  render 
the  high  tension  circuits  safe  to  handle,  because  there  is  a  reactance 
coil  shunted  across  the  key  which  permits  a  sufficient  How  of  current 
to  render  the  high-frequency  circuits  dangerous. 

It  will  be  noted  that  the  gap  contains  gaskets  of  two  colors,  gray 
and  red,  which  are  of  slightly  different  thicknesses,  the  gray  being 
thinner  than  the  red.  The  two  colors  are  to  be  interchanged  depend- 
ing on  whether  or  not  full  power  is  obtained  when  all  the  gaps  are 
used.  If  more  than  1  kilowatt  is  obtained,  substitute  a  gray  gasket  for 
a  red;  and,  vice  versa,  if  less  than  1  kilowatt  is  obtained,  substitute 
a  red  gasket  for  a  gray  one.  As  delivered  by  the  manufacturer  each 
gap  is  assembled  with  a  proper  number  and  kind  of  gasket  and  a  full 
set  of  spares  is  provided.  The  number  of  gaskets  in  place  will 
generally  be  correct,  but  on  account  of  small  variations  hi  spacing 
which  may  take  place  when  the  gap  is  opened  for  cleaning  it  may  be 
necessary  to  change  gaskets  from  one  color  to  the  other. 

The  gap  when  received  is  in  proper  condition  for  working  and 
should  not  be  opened  until  absolutely  necessary.  This  necessity  is 
made  evident  either  by  the  radiation  falling  below  its  usual  value 
when  the  circuits  are  properly  adjusted  or  by  inability  to  get  a  clear 
note  or  a  considerable  reduction  in  the  wattmeter  reading  when  the 
proper  number  of  plates  is  connected  up. 

Under  ordinary  conditions  it  ought  not  to  be  necessary  to  open  the 
gap  more  than  once  in  two  or  three  weeks,  and  in  case  of  any  trouble 
with  the  set  all  adjustments  should  be  gone  over  carefully  before 
opening  the  gap.  When  it  becomes  necessary  to  do  this,  loosen  the 
set  screw  in  the  end  of  the  gap  and  lift  out  the  plates.  It  will  prob- 
ably be  found  that  the  gaskets  and  plates  are  stuck  tightly  together. 
A  wrench  is  provided  for  breaking  the  plates  apart,  and  this  wrench 
is  to  go  over  the  gasket,  the  wrench  being  given  a  slight  twist 
until  the  plates  separate.  Do  not  twist  enough  to  damage  the  gasket. 
Any  irregularities  found  in  the  surface  of  the  plates  should  be 
smoothed  off  with  fine  emery  and  the  plates  wiped  perfectly  clean 
before  inserting  in  the  gap.  The  gaskets  are  expected  to  keep  the 
sparking  space  air-tight,  and  if  such  is  the  case  the  surfaces  of  the 
plates  wiU  be  found  to  have  a  bright  granulated  appearance.  If, 
however,  the  space  has  not  been  air-tight,  the  plates  will  show  black 
surfaces. 

In  case  the  gaskets  stick  so  tightly  that  opening  the  gap  tears  the 
surface  off  the  gasket,  the  plates  should  be  carefully  cleaned  and  a 
new  gasket  inserted.  The  tightening  bolt  of  the  gap  should  occasion- 
ally be  tried  to  see  that  it  is  perfectly  tight,  and  if  not  should  bo 
made  so. 


RADIOTELEGRAPH  Y.  91 

Owing  to  the  slight  compression  of  the  gaskets  which  takes  place 
in  time  it  will  probably  be  found  that  this  bolt  can  be  turned  from 
one-eighth  to  one-half  a  turn.  It  will  be  found  after  the  gap  has 
been  in  use  for  some  time  that  one  more  plate  will  have  to  be  con- 
nected in  for  full  power,  and  also  that  the  gap  improves  somewhat 
with  use  and  that  the  radiation  will  be  somewhat  higher  after  it  has 
been  in  service  for  a  short  period. 

If  when  the  gap  is  opened  a  plate  is  found  whose  sparking  surface 
is  partly  black  and  partly  bright,  it  is  not  an  indication  that  the 
gap  is  leaking  air,  but  that  this  particular  plate  may  not  have  been 
in  use  long  enough  to  consume  the  air  between  the  plates  when  first 
put  together.  Ordinarily  this  condition  will  be  found  only  on  the 
plates  which  are  not  in  use  at  all  times,  or  if  the  gap  is  opened  after 
being  in  use  only  a  short  time. 

After  the  plates  have  been  put  back  in  the  gap  the  set  screw  at  the 
end  should  be  tightened  up  again,  and  to  secure  air  tightness  it  should 
be  screwed  with  a  great  deal  of  pressure,  about  all  that  an  average 
man  can  exert  with  a  12-inch  monkey  wrench. 

The  base  of  the  quenched  gap  should  be  connected  to  ground,  a 
screw  in  the  base  being  provided  for  that  purpose. 

If  at  any  time  it  becomes  necessary  to  get  at  the  contacts  of  the 
oscillation  transformer  or  aerial  inductance,  set  the  instrument  on  the 
edge  of  a  table  with  the  slotted  side  of  the  base  overhanging.  Insert 
a  screw  driver  or  other  convenient  tool  in  one  of  the  holes  of  the 
perforated  cover  and  press  down  on  it,  when  the  cover  will  be  found  to 
slide  down  through  the  slot,  exposing  completely  the  coils  and  con- 
tacts. If  at  any  time  a  contact  appears  to  stick  at  the  spiral  con- 
ductor, it  can  be  lubricated  with  vaseline. 

In  case  it  is  desired  to  work  at  wave  lengths  other  than  those 
marked  on  the*  oscillation  transformer,  the  movable  coil  of  the  oscil- 
lation transformer  may  be  used  as  a  primary  and  the  fixed  coil  as 
secondary,  in  which  case  any  wave  length  up  to  the  limits  of  the 
apparatus  may  be  obtained.  When  using  this  arrangement  the 
switch  should.be  set  at  the  1200  meter  mark  for  medium  and  long 
wave  lengths,  and  at  whichever  of  the  other  positions  may  be  neces- 
sary for  the  shorter  wave  lengths.  The  adjustment  of  the  two  cir- 
cuits to  resonance  and  to  the  proper  coupling  should  be  made  as 
previously  described.  If  it  is  desired  to  work  at  less  than  400  meters, 
it  will  be  of  advantage  to  use  two  condenser  jars  instead  of  three 
and  to  substitute  red  gaskets  in  the  spark  gap  instead  of  gray  ones  to 
obtain  full  power. 

It  is  advisable  to  close  the  switch  short-circuiting  the  ammeter 
and  open  the  voltmeter  switch  on  the  board  after  the  set  has  been 
tuned  up,  as  it  protects  them  from  the  jerk  due  to  opening  and 
closing  the  key.  Owing  to  the  very  large  drop  in  voltage  when  the 


92  RADIOTELEGRAPH  Y. 

key  is  closed  the  reading  of  the  frequency  meter  may  not  be  very 
plain,  and  if  such  is  the  case  the  key  may  be  opened  and  the  first 
reed  which  starts  to  vibrate  after  opening  the  key~  indicates  the  fre- 
quency when  the  key  is  closed.  Usually,  however,  the  motion  of  the 
reed  is  sufficient  with  the  key  closed  except  when  working  at  reduced 
power.  It  is  possible  to  operate  at  any  power  between  |  and  1J 
kilowatts  by  cutting  in  circuit  the  right  number  of  plates  and  making 
proper  adjustment  of  the  generator  voltage. 

When  working  at  1  kilowatt,  with  proper  adjustment  of  all  cir- 
cuits, the  A.  C.  ammeter  will  read  between  10  and  11  amperes  and 
the  A.  C.  voltage  will  vary  between  125  and  150  volts  with  the  key 
closed.  The  power  factor  will  vary  between  80  and  85  per  cent.  All 
of  these  readings  will  vary  somewhat  with  the  wave  length  used, 
the  constants  of  the  particular  aerial  with  which  the  set  is  used,  and 
the  adjustments  made,  but  will  generally  be  within  the  limits  men- 
tioned. 

FIELD   WAGON    SETS. 

The  following  are  the  general  instructions  for  the  operation  and 
care  of  the  Telefunken  two-wagon  2-kilowatt  set: 

Engine. — The  engine  supplied  with  this  set  is  a  water-cooled,  single-cylinder  gas- 
oline engine  with  a  normal  speed  of  1,500  R.  P.  M.,  and  the  same  general  directions 
as  to  care  and  operation  which  apply  to  water-cooled  gasoline  engines  in  general 
apply  in  this  case,  and  the  principal  points  are  briefly  as  follows: 

Before  starting  make  sure — 

1.  That  the  water  tank  is  full. 

2.  That  all  bearings  have  been  oiled. 

3.  That  the  engine  has  sufficient  lubricating  oil  by  means  of  the  stopcock  on  under 
part  of  crank  case.     If  it  drips  when  opened,  there  is  sufficient  oil. 

4.  That  there  is  sufficient  gasoline  in  the  tank  as  indicated  by  the  gauge  on  the 
front  of  the  tank. 

5.  That  the  main  switch  of  the  generator  is  open. 
To  start— 

1.  Open  gasoline  feed  cock. 

2.  Prime  carburetor  by  plunger  on  top. 

3.  Set  the  governor  control  handle  (just  above  the  crank)  vertically,  i.  e.,  halfway 
across  the  scale. 

4.  Set  the  spark-control  lever  on  the  magneto  on  bottom  notch. 

5.  Crank. 
After  starting — 

1.  Make  sure  that  the  fan  is  running. 

2.  Close  main  switch. 

Speed:  The  speed,  as  indicated  by  the  tachometer  on  the  engine,  is  controlled  by 
the  position  of  the  governor  control  handle  (directly  over  the  crank)  and  by  the  posi- 
tion of  the  spark-control  lever  on  the  magneto  (at  the  right),  and  the  best  position 
of  each  for  any  particular  speed  is  best  and  easily  determined  by  experiment. 

To  shut  down  temporarily — 

1.  Open  main  switch  of  generator. 

2.  Press  button  on  front  of  magneto  until  engine  stops. 


EADIOTELEGKAPHY. 


93 


To  shut  down  permanently — 

1.  Same  as  above. 

2.  Ditto. 

3.  Turn  off  gasoline. 

4.  In  cold  weather  empty  all  water  out  of  every  part  of  cooling  system  by  means 
of  the  cocks  provided  for  that  purpose. 

Generator. — The  alternating-current  generator  supplied  with  this  set  is  of  the  induc- 
tor type  with  the  field  and  armature  winding  stationary,  and  has  therefore  no  brushes 
or  sliding  contacts  of  any  kind.  Its  normal  voltage  is  85.  The  exciter  is  an  ordinary 
low-voltage  direct-current  machine.  The  voltage  of  the  alternating-current  genera- 
tor is  varied  by  means  of  the  rheostat  in  series  with  its  field.  The  rheostat  is  located 
in  the  lower  left-hand  corner  of  the  front  part  of  the  instrument  wagon.  The  connec- 
tions between  the  power  wagon  and  the  instrument  wagon  are  made  by  means  of  a 
flexible  armored  four-conductor  cable  having  the  sockets  so  arranged  that  the  ter- 
minals can  be  inserted  only  in  the  proper  manner,  the  circuits  of  the  alternator, 
exciter,  etc.,  being  shown  in  figure  78. 

Transmitter  and  receiver. — The  connections  of  both  are  clearly  shown  in  the  drawing 
and  require  no  further  description. 

To  adjust  the  transmitter  for  any  wave  length  within  the  range  of  the  set  proceed 
as  follows,  assuming  that  the  desired  wave  length  is  1,000  meters: 

1.  If  it  is  intended  to  send  at  full  power,  adjust  the  voltage  of  the  generator  by 
means  of  the  slide  rheostat  (at  the  left)  to  about  85  volts. 

2.  If  it  is  intended  to  send  at  less  than  full  power,  short-circuit  one  or  more  of  the 
gaps  by  means  of  the  clips  provided  and  at  the  same  time  reduce  the  generator  voltage 
about  10  per  cent  per  gap  short-circuited. 

3.  Set  the  primary  variometer  (at  the  left)  at  the  wave  length  desired,  viz,  1,000. 

4.  Put  the  aerial-coil  plug  (at  the  right)  in  hole  No.  1,  marked  680/1050.    This 
adds  sufficient  inductance  to  the  aerial  to  bring  the  final  adjustment  within  range  of 
the  aerial  variometer. 

5.  Make  the  final  adjustment  with  the  aerial  variometer  (also  on  the  right  and  on 
one  side  of  the  aerial  coils)  by  turning  it  slowly  up  from  zero  until  the  ammeter  in 
the  aerial  ground  circuit  indicates  a  maximum. 

6.  The  transmitter  is  now  adjusted  for  the  most  efficient  production  and  radiation 
of  the  wave  length  selected  when  used  with  the  aerial  and  counterpoise  supplied 
with  the  set. 

Receiver. — To  receive,  close  the  large  double-pole  switch  at  the  top  of  the  receiver. 

The  plug  holes  marked  with  roman  numbers  (at  the  right  on  the  receiver)  are  con- 
nected to  taps  on  the  aerial  or  primary  coil.  The  wave  range  of  this  coil  is  approxi- 
mately as  follows,  with  a  proper  aerial: 


Plug. 

Condenser  switch  at— 

Short  waves. 

Long  waves. 

I  
II... 

Meters. 
260-400 
310-510 
370-730 
-          540-1,  060 

Meters. 
500-600 
640-910 
900-1,410 
1,270-2,150 
1,860-3,080 
2,700-4,000 

III... 

IV 

V... 

VI.                ... 

The  turns  on  the  detector  or  loose  coupling  coil  are  variable  by  means  of  the  switch 
located  on  its  top,  the  wave  range  for  each  tap  being  marked. 

Either  of  the  two  detectors  can  be  used  by  means  of  the  switch  located  between 
them. 


94  RADIOTELEGBAPHY. 

For  receiving  a  signal  of  a  known  wave  length  the  following  procedure  can  be 
recommended : 

1.  Use  tight  coupling. 

2.  Plug  in  on  the  aerial  coil. 

3.  Set  the  switch  on  the  detector  coil  at  about  ">l=500/1000." 

4.  Turn  the  condenser  very  slowly  over  the  entire  scale. 

5.  Change  the  plug  on  aerial  coil  and  repeat  No.  4.     When  signals  are  finally  heard, 
the  coupling  and  the  position  of  the  switch  on  the  detector  coil  are  varied  until  the 
best  results  are  obtained. 

NOTE. — In  some  cases  two  combinations  of  the  aerial  plug  and  condenser  give 
almost  equally  good  results.  The  best  one  is  that  in  which  the  larger  part  of  the 
condenser  is  used  with  condenser  switch  at  "short  waves"  and  vice  versa,  with  the 
condenser  switch  at  "long  waves."  The  aerial  used  with  this  set  should  have  a 
capacity  of  0.0011  mf  and  a  natural  period  of  450  meters. 

The  following  detailed  notes  on  the  circuits  and  operation  of  the 
set  have  been  found  useful  as  a  result  of  actual  work  in  the  field : 

POWER    CIRCUITS. 

Referring  to  connection  diagram  76,  it  is  seen  that  D.  C.  leads 
marked  3  and  4  go  to  both  receiving  switches  in  series.  It  is  therefore 
necessary  to  have  the  main  switches  of  both  receiving  sets  in  the  same 
position — that  is,  cut  off — when  sending,  even  though  one  receiving 
set  may  have  no  aerial  wire  connected  to  it.  A  flash  due  to  the  break- 
ing of  this  D.  C.  circuit  will  be  seen  at  the  rotary  switch  if  the  receiv- 
ing set  is  cut  in  before  the  engine  is  stopped.  The  large  double-pole 
switch  at  the  top  of  the  receiver  when  closed  so  as  to  connect  the 
receiver  to  the  aerial  and  counterpoise  automatically  disconnects  the 
sending  side  from  the  aerial  and  counterpoise.  This  feature  is  not 
indicated  in  the  diagram  of  connections  where^the  receiving  set  when 
cut  in  is  apparently  shunted  by  the  sending  set. 

TRANSFORMER    PRIMARY    CIRCUIT. 

From  A.  C.  lead  No.  1  to  the  primary  inductance,  to  the  snap 
switch,  to  the  ammeter,  to  the  primary  of  the  transformer,  to  the  key, 
and  via  A.  C.  lead  No.  2  back  to  the  generator.  The  voltmeter  is 
across  the  A.  C.  leads  as  shown.  If  the  voltmeter  shows  voltage,  but 
upon  closing  the  key  no  spark  takes  place  at  the  spark  gap,  the  snap 
switch  in  the  primary  circuit  is  probably  open. 

The  voltage,  as  indicated  by  the  voltmeter,  must  never  be  more  than 
85.  If  it  is  desired  to  change  the  generator  frequency  (and  the 
pitch  of  the  note  emitted),  in  order  to  secure  greater  selectivity  for 
the  set  when  working  in  the  presence  of  other  sets  having  about  the 
same  generator  frequency,  the  engine  may  be  slowed  down  or  speeded 
up,  but  the  drop  or  rise  in  voltage  incident  thereto  must  be  compen- 
sated for  by  a  change  in  the  generator  rheostat,  so  that  the  voltage 
will  be  kept  constant  at  85  when  using  all  the  gaps  of  the  spark  gap. 
Any  violation  of  this  rule  will  cause  a  breakdown  in  the  transformer. 


EADIOTELEGEAPHY. 


95 


HIGH-FREQUENCY    CIRCUITS — TRANSMITTER . 

Closed  oscillating  circuit. — This*  consists   of  the   condenser,   vari- 
ometer; and  spark  gap.     It  is  to  be  noted  that  the  variometer  is  com- 


FIG.  76. 


mon  to  both  closed  and  open  oscillatory  circuits,  and,  therefore,  that 
changing  the  variometer  (which  is  the  one  at  the  left-hand  side  of  the 


96 


RADIOTELEGRAPHY. 


chest  and  has  scale  divisions  in  wave  lengths  marked  upon  it)  not 
only  changes  the  period  to  which  the  closed  oscillatory  circuit  is 
tuned,  but  also  slightly  changes  the  tuning  of  the  open  oscillatory  cir- 
cuit. A  word  of  caution  should  be  given  concerning  the  switch  marked 
"Little"  and  "Great"  which  throws  the  coils  of  this  variometer 
from  a  parallel  to  a  series  connection  or  vice  versa.  This  switch  can 
only  be  moved  to  the  right  or  left — to  "Little"  or  to  "Great" — when 
the  index  is  directly  opposite  to  the  dividing  line  between  the  red  and 


Condenser 


Connections  for  Sending 

FIG.  77. 

the  white  divisions.  Any  attempt  to  throw  this  switch  when  the  vari- 
ometer coils  are  in  any  other  position  will  only  result  in  damage  to  the 
switch. 

OPEN    OSCILLATORY    CIRCUIT. 

This  consists  of  the  aerial,  aerial  or  loading  coils,  plug  for  cutting 
in  proper  coil,  the  aerial  variometer  (marked  from  zero  to  180°),  the 
variometer  common  to  both  closed  and  open  oscillatory  circuits,  the 
hot-wire  ammeter,  and  the  counterpoise  or  ground. 


BADIOTELEGRAPHY. 


97 


The  antenna  supplied  by  the  Signal  Corps  for  this  set  has  a  natural 
wave  length  of  450  meters  and  a  capacity  of  about  0.0011  mf. 

It  is  found  by  experiment  that  the  set  using  the  Signal  Corps  80- 
foot  mast  and  rubber-covered  counterpoise  works  best  at  about  1,000 
meters,  where  the  antenna  hot-wire  ammeter  reads  about  7J  amperes. 


CODING    OF   WAVE    LENGTHS. 


The  great  advantage  of  this  set  lies  in  the  fact  that  any  desired 
wave  length  from  675  to  2,220  meters  can  be  sent  out  at  will,  and  if 
the  wave  length  is  changed  after  every  word  of  a  message,  according 
to  a  prearranged  code  of  wave  lengths — for  example,  the  first  word 
sent  with  700  meters,  the  next  with  2,100,  the  next  with  1,400,  etc. — it 


hrlH 

Protective  devices 


FIG.  78. 

will  be  difficult  for  any  eavesdropping  operator  who  has  not  the  wave- 
length code  to  follow  the  changes  of  wave  length  with  any  success. 
Hence,  messages  may  sometimes  be  kept  confidential  even  when  sent  fn 
plain  English.  This  will  take  considerable  drill  on  the  part  of  two 
men,  the  operator  and  an  assistant,  who  will  rapidly  make  the  neces- 
sary changes  in  the  loading  coils  and  variometers  at  a  signal  from 
the  operator. 

The  first  step  will  be  to  make  experimental  determination  of  the 
combinations  of  loading  coils  and  variometers  necessary  to  produce 
the  best  radiation  for  every  wave  length  within  the  range  of  the  set 
and  to  set  them  down  in  the  form  of  a  table.  Thus,  starting  .with  700 
meters,  put  the  left-hand  variometer  at  700,  put  the  plug  in  the  hole 
92448—15 7 


98 


RADIOTELEGRAPHY. 


marked  675-1,080,  and  then  slowly  move  the  aerial  variometer  from 
0°  toward  180°  until  the  hot-wire  ammeter  shows  the  best  radiation. 
The  various  adjustments  can  then  be  noted  in  a  table  for  future  ref- 
erence, thus:  (The  figures  given  are  not  the  actual  figures.  These 
must  be  determined  for  each  set  separately.) 

TABLE  I. 


Wave 
length. 

Variom- 
eter. 

Loading 
coil. 

Aerial 
variometer. 

Amperes  on 
hot  wire. 

700 

700 

675-1,080 

12 

6.9 

750 

750 

675-1,080 

20 

6.95 

800 

800 

675-1,080 

50 

7 

850 

850 

675-1,080 

80 

7.05 

900 

900 

675-1,080 

120 

7.1 

950 

950 

920-1,310 

4 

7.15 

1,000 

1,000 

920-1,310 

10 

7.25 

1,050 

1,050 

920-1,310 

60 

7 

1,100 

1,100 

920-1,310 

90 

6.8 

1,150 

1,150 

920-1,  310 

105 

6.6 

1,200 

1,200 

920-1,310 

130 

6.4 

1,250 

1,250 

1,240-1,510 

5 

6.2 

and  so  on,  finding  the  best  combination  for  every  50  meters  increase 
in  wave  length  up  to  the  limit  of  the  set. 

LIMITATIONS    OF    SYSTEM    OF    CODING   WAVE    LENGTHS. 

It  will  be  noted  that  there  is  one  best  wave  for  the  set,  namely, 
about  1,000  meters.  From  some  experiments  made  recently  at  Fort 
Leavenworth  it  is  concluded  that  it  is  safe  to  state  that,  up  to  about 
75  miles  over  average  land,  the  falling  off  of  energy  due  to  the  use  of 
the  longest  wave  lengths  will  not  be  so  great  as  to  prevent  the  use  of 
any' wave  length  within  the  limits  of  the  set  (675-2,220  meters),  but 
that  beyond  that  distance,  up  to  the  extreme  daylight*  distance  of  the 
set  (about  185  miles)  j  it  would  be  safer  not  to  work  with  any  wave 
length  greater  than  1,800  meters. 

Only  further  experiments  in  the  field,  between  two  similar  sets 
working  at  gradually  increasing  long  ranges,  will  determine  the 
greatest  distance  at  which  the  whole  scale  of  sending  wave  lengths 
may  be  used. 

From  the  table  plotted  as  above  different  codes  of  wave  lengths, 
differing  by  many  meters  from  each  other,  may  be  agreed  upon,  to 
be  changed  daily  in  actual  work,  and  confided  to  all  operators  con- 
cerned. 

RECEIVING   CIRCUITS. 

Primary  or  aerial  circuit. — One  lead  from  aerial  comes  through 
combination  switch  to  the  primary  of  the  transformer  (shown  on 
the  left  *of  figure  79),  from  there  through  plug  contact  to  a  point 
on  the  little  switch  marked  "Long  waves "-" Short  waves";  and,  if 


EADIOTELEGKAPHY. 


99 


the  switch  is  thrown  to  the  long-wave  side,  the  circuit  goes  direct  to 
the  ground;  the  variable  condenser  being  then  in  parallel  with  the 
primary  of  the  transformer.  If  the  switch  is  thrown  to  the  short- 
wave side,  the  variable  condenser  is  in  series  with  the  aerial,  the 
primary  of  the  condenser,  receiving  transformer,  and  the  counter- 
poise or  ground. 

The  secondary  or  detector  circuit  consists  of  the  secondary  of  the 
transformer  in  series  with  the  usual  stopping  condenser,  connected 
through  the  main  switch  to  the  detectors.  The  telephones  are  in 
shunt  to  the  stopping  condenser. 

The  detector  supplied  is  of  the  iron  pyrites  variety,  which  lacks 
the  sensitiveness  of  the  steel-wire  rough-silicon  detector  of  the  Signal 
Corps  type,  or  of  the  Perikon.  Any  other  detector  may  easily  be 


Variab/e 
Condenser 


(O)       (O) 
Telephone 
Connections  for  Receiving 

FIG.  79. 

substituted  for  the  detectors  supplied  with  the  set,,  the  range  of 
which  may  be  thereby  easily  increased. 

With  the  switch  thrown  to  "Long  waves"  the  operator  will  get 
the  best  results  when  using  a  small  number  of  degrees  of  the  variable 
condenser  and  as  large  primary  as  possible,  and,  vice  versa,  with  the 
switch  to  "  Short  waves,"  which  places  the  variable  condenser  in 
series  with  the  primary  coils.  The  largest  possible  amount  of  capacity 
of  the  variable  condenser  and  the  smallest  amount  of  primary 
inductance  should  be  used  for  maxiumum  strength  of  signals. 

The  combination  switch  which  is  used  primarily  to  cut  the  receiv- 
ing set  onto  the  antenna  and  counterpoise  simultaneously  performs 
several  operations.  Opening  this  switch  disconnects  the  receiving 
set  from  the  antenna  and  counterpoise ;  automatically  connects  send- 
ing set  to  the  aerial  and  counterpoise;  closes  D.  C.  circuit  of  genera- 


100  RADIOTELEGRAPH  Y. 

tor;  disconnects  detectors  from  secondary  of  receiving  transformer, 
thus  opening  that  circuit  and  preventing  detectors  from  being  af- 
fected by  the  spark  when  sending,  and  also  opens  the  primary  cir- 
cuit of  the  receiving  transformer.  As  the  limits  of  the  various  coils 
of  the  primary  and  secondary  are  marked,  there  should  be  no  diffi- 
culty about  setting  the  receiving  apparatus  approximately  for  the 
wave  length  of  a  station  whose  wave  length  is  known.  The  operator 
then  varies  his  condenser,  and  also  the  coupling  between  the  primary 
and  secondary  of  the  receiving  transformer,  until  he  gets  the  best 
adjustment.  Changing  the  coupling  (that  is,  puFing  the  secondary 
away  from  or  pushing  it  closer  to  the  primary)  changes  the  wave 
length ,  though  to  not  as  great  an  extent  as  does  varying  the  con- 
denser. Some  stations  can  not  be  heard  at  all  well  unless  the  secon- 
dary coil  is  pulled  some  distance  away  from  the  primary.  Practice 
is  the  best  guide  to  a  working  knowledge  of  the  tuning  of  the  receiv- 
ing set. 

Figure  77  shows  simplified  schematic  diagram  of  the  transmitting 
circuits.  Figure  78  shows  the  generator  circuits. 

CALIBRATION    IN    WAVE    LENGTHS. 

The  receiving  set  should  be  calibrated  so  as  to  locate  the  actual 
combinations  necessary  for  receiving  the  wave  lengths  sent  out  by  a 
similar  sending  set,  either  by  actual  tuning  to  another  set  sending 
out  successive  wave  lengths  differing  from  each  other  by  50  meters, 
as  outlined  above,  or  by  using  the  wave  meter  provided  with  each 
wagon  set  as  a  sending  device,  and  with  its  coupling  coil  held  near 
the  antenna  lead,  set  up,  consecutively,  different  wave  lengths  in  the 
antenna  and  make  adjustments  of  receiving  set  necessary  to  tune  to 
the  particular  wave  lengths  sent  out;  then  compile  a  table  showing 
adjustments  of  condenser  switch,  primary,  secondary,  and  variable 
condenser  necessary  for  each  wave  length  in  turn,  so  that  the  receiv- 
ing operator  can  at  once  adjust  his  receiving  apparatus  to  any  desired 
wave  length,  and,  by  quick  changes,  constantly  follow,  according  to 
prearranged  code,  the  message  sent  out  by  the  other  station. 

It  is  recommended  that,  hi  order  to  eliminate  one  adjustment  of 
the  receiving  set,  the  primary  and  secondary  of  the  receiving  trans- 
former be  kept  in  the  same  relative  positions  throughout;  that  is, 
as  close  to  each  other  as  possible.  This,  while  possibly  sacrificing 
efficiency,  secures  simplicity.  The  receiving  operator's  chart  may  be 
arranged  as  follows: 

Best  receiving  adjustments  necessary  to  tune  to  wave  lengths  used 
by  similar  wagon-set  sending  wave  lengths  shown  in  Table  I. 


KADIOTELEGRAPHY. 
TABLE  II.    ' 


101 


Wave 
length. 

Switch. 

Primary. 

Secondary. 

Condenser. 

700 

Short  waves  

370-730 

500-1,000 

80° 

750 

Long  waves  

640-910 

500-1,000 

40° 

NOTE.— The  condenser  adjustments  given  above  are  not  the  actual  ones  necessary  for  wave  lengths 
given. 

And  so  fourth  for  every  50  meters. 

Constant  drill  in  changing  sending  and  receiving  adjustments,  car- 
ried on  between  two  or  more  similar  sets,  will  result  in  remarkable 
efficiency  and  rapidity,  and  the  time  necessary  for  transmission  of 
messages  will  be  found  to  be  but  little  increased  over  that  required 
when  sending  on  a  single  wave  length. 

RECEIVING    BY    CODING   OF    WAVE    LENGTHS. 

Two  complete  receiving  sets  are  provided  with  each  wagon  set, 
though  ordinarily  only  one  is  used.  Two  messages  from  different 
stations  may  be  copied  from  the  same  antenna  without  either  operator 
hearing  the  message  copied  by  the  other.  To  do  this  it  is  of  course 
necessary  to  have  a  lead  from  the  aerial  running  to  each  of  the  receiv- 
ing sets.  A  change  in  the  tuning  of  one  receiving  set  will  call  for  a 
slight  readjustment  of  the  other  receiving  set,  however,  in  order  that 
the  latter  set  may  stay  in  tune  with  the  given  wave  length. 

The  use  of  two  receiving  sets  in  parallel  makes  it  comparatively 
simple  to  follow  a  message  sent  according  to  a  prearranged  code  of 
wave  lengths,  for  it  is  perfectly  practicable  to  so  arrange  the  wave- 
length code  that  the  waves  of  any  length  within  certain  limits  will 
fall  within  the  limits  of  the  condenser  of  either  one  set  or  the  other, 
and  either  one  operator  or  the  other,  without  making  any  change  of 
adjustment  other  than  a  mere  movement  of  the  condenser  handle, 
will  have  his  apparatus  constantly  in  resonance  with  the  incoming 
waves. 

Thus  let  us  say  that  in  the  code  agreed  upon,  which  includes  all 
wave  lengths  between  900  and  2,150  meters,  the  first  word  will  be 
sent  with  a  900-meter  wave,  the  next  with  2,100,  followed  by  1,500, 
1,850,  1,050,  2,000,  etc. 

The  two  sets  are  cut  in  at  the  receiving  station  and  are  each  manned 
by  an  operator.  Operator  No.  1,  at  the  left,  puts  the  plug  in  the  hole 
of  the  primary  of  his  receiving  set  marked  "  900-1410,"  couples  his 
primary  and  secondary  as  closely  as  possible,  throws  his  receiving 
switch  to  "Long  waves/'  and  puts  the  switch  of  the  detector  coil  on 
whatever  coil  will  give  him  the  strongest  signals.  He  can  then,  by 
merely  moving  his  condenser  from  0°  toward  180°,  tune  his  set  to 


102  BADIOTELEGRAPHY. 

any  dosired_  wave  botween.900  and  1,410  meters,  and  it  will  be  his 
duty  to  copy  all  words  of  the  message  which  may  fall  within  those 
limits. 

Operator  No.  2,  on  the  right,  similarly  throws  his  switch  to  "Long 
waves"  and  plugs  in  primary  coil  marked  "1270-2150,"  and  makes 
the  other  adjustments  as  given  for  No.  1.  He  is  then  ready  to  receive 
any  wave  between  1,270  and  2,150  meters  by  merely  setting  the 
pointer  of  his  condenser  at  the  proper  number  of  degrees  on  the 
condenser. 

From  Table  II,  prepared  as  before  described,  either  operator  can 
set  his  condenser  accurately  and  instantly  to  the  proper  reading  for 
any  desired  wave  length  within  limits;  hence,  when  the  message 
is  to  be  received,  the  first  word  sent  as  per  schedule  at  900  meters 
is  copied  by  No.  1  operator,  who  has  his  pointer  at  the  proper  place 
on  the  condenser  scale;  the  second  word  at  2,100  meters  by  No.  2, 
who  has  already  set  his  pointer  at  the  proper  place.  As  the  third 
word  is  sent  at  1,500  meters,  No.  2  readjusts  his  condenser  for  the 
next  word,  and  later  turns  the  pointer  to  the  proper  place  for  the 
next  word  at  1,850;  then  No.  1  comes  in  on  his  set  and  copies  the 
next  word  at  1,050  meters,  No.  2  the  next  at  2,000,  and  so  forth,  the 
words  being  placed  together  in  accordance  with  the  order  of  their 
receipt  so  as  to  make  a  complete  message. 

This  method  of  using  two  operators  saves  time  by  dispensing 
with  a  number  of  switch  and  plug  changes,  which  a  single  operator 
would  have  to  make  in  using  only  one  receiving  set. 

The  method  of  using  two  receiving  sets  tuned  as  above  could 
easily  be  worked  by  one  operator  who  could  wear  the  single  head 
receiver  of  one  set  on  one  ear  and  that  of  the  other  on  his  other  ear. 

All  these  methods  should  be  practiced  continually  to  improve  the 
skill  of  the  operators. 

Care  must  be  taken  to  close  or  open  both  main  switches  of  the 
receiving  set  at  the  same  time  when  working  both  receiving  sets, 
in  order  to  prevent  sending  into  one  of  the  receiving  sets  and  burning 
it  out. 

PACK  SETS. 

Radio  pack  sets  will  hereafter  be  accounted  for  as  "Radio  pack 
sets,  complete,"  and  will  consist  of  the  following  parts: 


Operating  chest,  containing: 
1  transformer. 
1  condenser. 

1  oscillation  transformer. 
1  sending  key. 
1  spark  gap. 
1  hot  wire  ammeter. 
1  switch. 


Operating  chest,  containing — Contd. 
receiving  set. 

connecting    cord    for    generators 
(4-condr.). 

double-head  receiver, 
test  buzzer, 
tool  kit. 
extra  set  crvstals. 


RADIOTELEGKAPHY. 


103 


Mast,  type  F,  consisting  of — Contd. 
1  antenna. 

1  counterpoise. 
9  carriers,  wire. 
4  pins,  antenna. 

2  hammers. 

1  set  adapters  for  tent  (4  pieces). 
1  bag,  antenna,  and  counterpoise. 
1  bag,  accessory. 
Pack  frames,  consisting  of: 

3  frames  (1  set).    Each  frame  is  com- 
plete with  cincha,  2  straps  with 
snap  hooks,  and  2  plate  staples. 

Tent,  consisting  of: 

1  tent. 
14  pins. 

2  guy  ropes. 

1  insulating  device. 


Operating  chest,  containing — Contd. 

1  canvas  case  for  receiver. 

2  connectors,  2-wire  (lower  half),  an- 
tenna and  counterpoise. 

1  connector,  4-wire  (lower  half),  gen- 
erator. 

1  copy    "Radio telegraphy"    (S.    O. 
Circ.  No.  1,  1914). 

Hand  generator,  consisting  of: 

2  cranks. 
1  otand. 

1  speedometer  (carried  in  operating 

chest). 

1  canvas  cover. 

1  cap  for  speedometer  opening. 
Mast,  type  F,  consisting  of: 
1  top  section. 
1  bottom  section. 
8  intermediate  sections. 
4    intermediate    sections    (3    extra 
pieces). 

Incomplete  sets  will  be  designated  as  "  Radio  pack  sets,  complete," 
except  the  items  not  on  hand,  which  will  be  enumerated. 

The  1913  model  Signal  Corps  field  radio  pack  set  is  of  the  500- 
cycle  quenched-spark  type,  similar  to  the  1912  model  described  in 
Signal  Corps  Bulletin  No.  17,  except  that  certain  improvements  have 
been  made  whereby  the  output  has  been  doubled  and  the  range  of 
operation  considerably  increased. 

SECTIONAL   MAST. 

The  new  type  F  sectional  mast  with  short  sections  will  supersede 
the  type  D  in  use  as  soon  as  the  stock  of  the  latter  now  on  hand  be- 
comes exhausted.  The  type  F  mast  equipment  consists  of  14  sections, 
each  4  feet  2  inches  long  or  5  feet  2  inches  over  all,  including  the 
coupling  tube.  The  10  sections  are  used  for  the  mast  itself,  3  sections 
for  the  shelter  tent  when  erected,  and  1  extra  section  for  use  in  case 
one  of  the  others  becomes  unserviceable. 

The  mast  can  be  erected  from  the  ground  and  more  easily  with 
the  short  sections  than  with  the  long  sections. 

When  starting  to  erect  the  mast,  the  four  antenna  wires  and  guys 
should  be  laid  out  on  the  ground  at  right  angles  to  each  other  and 
the  umbrella  insulator  put  on  the  upper  end  of  the  section  not  pro- 
vided with  a  coupling  tube.  This  section  should  then  be  raised  and 
eight  more  sections  with  coupling  tubes  added,  section  by  section, 
the  tenth  and  last  section  being  the  one  provided  with  the  insulator 
fixed  at  the  bottom  end.  During  the  erection  the  mast  should  be 
kept  as  nearly  vertical  as  possible  by  the  men  holding  the  distant 
ends  of  the  antenna  guy  ropes.  Owing  to  the  liability  to  buckle,  no 


104  RADIOTELEGRAPH  Y. 

attempt  should  be  made  to  erect  the  entire  mast  at  one.  time;  that  is, 
by  coupling  all  sections  together  and  raising  by  means  of  the  guys. 

ANTENNA   AND   COUNTERPOISE. 

The  standard  antenna  is  of  the  umbrella  type  with  four  radiating 
wires,  each  85  feet  long,  suitably  insulated  at  the  open  ends  and  held 
as  nearly  horizontal  as  possible  by  guy  rope  extensions,  each  85  feet 
long,  the  outer  ends  of  which  are  made  fast  to  ground  pins.  The 
standard  counterpoise  has  four  radiating  insulated  wires,  each  100 
feet  long,  laid  out  on  the  ground  under  the  antenna  wires.  Both 
antenna  and  counterpoise  wires  are  carried  on  hand  reels  for  conven- 
ience in  packing  and  quick  unreeling  in  setting  up  the  -mast. 

GENERATOR. 

The  generator  is  a  hand-driven,  18-pole,  alternating-current  ma- 
chine having  an  intermittent  output  of  250  watts  at  110  volts  and 
500  cycles  at  a  speed  of  3333  R.  P.  M.  It  is  self -excited,  the  exciting 
current  for  the  fields  being  generated  by  a  small  shunt-wound  direct- 
current  machine,  the  armature  of  which  is  mounted  on  the  same  shaft 
as  the  alternator  armature.  The  exciter  has  two  poles  and  delivers 
the  direct  current  at  about  110  to  150  volts.  The  whole  machine  is 
driven  by  two  handles,  which  should  be  turned  at  the  rate  of  33 
R..P.  M.  to  give  the  necessary  armature  speed  of  3333  R.  P.  M.,  the 
combination  gear  having  a  ratio  of  about  100  to  1.  The  direction 
of  rotation  of  the  handles  must  be  as  shown  by  the  arrow  on  the  top 
of  the  gear  case,  as  otherwise  the  machine  will  not  deliver  any  cur- 
rent. The  whole  is  inclosed  in  a  dust-proof  aluminum  case.  To 
obtain  access  to  the  commutator,  remove  the  flywheel,  taking  care 
not  to  lose  the  key  on  the  flywheel  shaft;  then  remove  the  large 
brass  nut  and  the  aluminum  disk  held  in  place  by  the  latter,  after 
which  it  will  be  found  that  the  commutator  is  readily  accessible. 
To  remove  the  armature  from  the  machine,  proceed  as  above;  then 
take  off  the  casing  covering,  the  spur  gears  at  the  opposite  end  of 
the  shaft,  and  the  gears  themselves;  before  removing  the  armature 
take  the  brushes  out  of  the  holders  to  avoid  injuring  or  breaking 
them. 

SPEED    INDICATOR. 

A  speed  indicator  is  mounted  on  the  upper  part  of  the  gear  case 
in  sight  of  the  men  driving  the  machine  so  as  to  show  if  it  is  being 
driven  at  the  proper  speed,  at  which  time  the  red  line  on  the  moving 
vane  coincides  with  the  black  index  or  arrow  at  the  window.  The 
vane  is  divided  diagonally  into  black  and  white  parts,  the  white 
showing  if  the  speed  is  too  low  and  the  black  if  too  high. 


KADIOTELEGRAPHY.  105 

In  putting  the  speed  indicator  in  place  it  may  be  necessary  to  turn 
handles  slightly  so  as  to  permit  the  gears  to  engage. 

In  case  the  vane  of  the  speed  indicator  conies  on  the  underside 
when  the  indicator  is  screwed  into  place,  it  can  be  turned  into  proper 
position  after  loosening  the  depressed  set  screw  on  the  threaded  part 
fitting  into  the  case  and  then  tightening  the  set  screw  again. 

The  gearing  is  a  combination  planetary  worm-and-spur  type  of 
high  efficiency  when  in  proper  alignment.  The  high-speed  shafts 
have  ball  bearings  and  the  gears  run  in  oil,  so  as  to  reduce  the  fric- 
tion as  much  as  possible.  The  gears  should  never  be  taken  apart 
unless  absolutely  necessary  to  replace  worn  or  broken  parts,  and 
then  only  by  an  experienced  person.  If  not  properly  reassembled, 
or  if  the  driving  wheel  does  not  run  perfectly  true  with  the  worm, 
undue  friction  and  wear  will  result,  the  machine  will  be  harder  to 
turn  than  before,  and  the  gears  will  be  speedily  destroyed. 

No  grease  should  be  used  on  the  gears,  but  only  a  light,  thin  oil, 
such  as  Medium  Monogram,  which  must  be  kept  free  from  acid  and 
water,  both  of  which  will  rust  the  ball  bearings.  The  oil  should  be 
supplied  through  a  small  cap  on  the  opposite  side  of  the  case  from 
the  speed  indicator.  The  level  should  be  kept  not  more  than  one- 
eighth  inch  above  the  lower  edge  of  the  glass  window  at  the  flywheel 
end  of  the  gear  case;  if  kept  above  this,  the  oil  will  overflow  to  the 
lower  part  of  the  case  and  cause  trouble  and  sparking  at  the  com- 
mutator and  collector  rings.  The  same  kind  of  oil  should  be  used 
on  the  flywheel  shaft  through  the  small  hole  on  the  upper  side  of  the 
bearing. 

With  the  exception  of  an  occasional  addition  of  oil,  the  machine 
should  run  for  months  without  attention.  If  the  oil  becomes  thick 
or  dirty,  the  gearing  should  be  washed  out  with  gasoline  and  refilled 
with  clean  oil  without  dismantling. 

The  tension  on  both  sets  of  the  generator  brushes  should  be  kept 
as  light  as  possible  consistent  with  good  commutation.  A  small 
increase  in  the  friction  of  these  brushes  will  require  considerable 
additional  power  to  drive  the  machine.  Both  sets  of  brushes  can  be 
removed  when  necessary  through  openings  in  the  lower  part  of  the 
case,  the  D.  C.  exciter  brushes  being  at  the  flywheel  end  and  the  A.  C. 
brushes  at  the  opposite  end. 

Care  must  be  taken  not  to  start  or  stop  the  machine  suddenly,  as 
this  may  strain  or  break  the  gears.  The  machine  must  not  be 
stopped  by  means  of  the  handles,  but  only  by  friction  on  the  flywheel. 

The  leads  from  the  armature  of  the  A.  C.  generator  are  directly 
connected  to  the  transformer  primary  by  means  of  the  heavy  pair 
of  leads,  the  larger  plug  of  which  being  put  into  the  socket  at  the 
left-hand  end  of  the  operating  chest  marked  "Gen."  and  the  smaller 
plug  into  the  socket  on  the  underside  of  the  gear  case,  also  marked 


106  RADIOTELEGKAPHY. 

"Gen."  The  sending  key  is  in  the  circuit  of  the  alternator  fields 
and  the  exciter  armature,  and  is  so  connected  by  means  of  the 
light  pair  of  leads,  the  larger  plug  of  which  being  put  into  the  socket 
at  the  left  end  of  the  chest  marked  "Fid."  and  the  smaller  plug  into 
the  socket  on  the  underside  of  the  case,  also  marked  "Fid."  By 
the  use  of  these  circuits  shown,  the  electrical  load  on  the  machine 
is  limited  to  the  small  one  of  the  exciter  field,  except  when  the  key 
is  closed  in  sending.  Experiments  have  shown  that  twice  the  out- 
put of  the  former  machines  can  thus  be  obtained  with  practically 
no  more  tiring  effects  on  the  men  than  before. 

A  canvas  cover  is  provided  for  the  generator,  which  should  be 
kept  on  at  all  times  when  the  generator  is  not  in  use. 

In  making  the  kit  ready  for  transportation,  the  speed  indicator 
should  be  removed  and  packed  in  its  proper  place  in  the  operating 
chest  and  the  opening  closed  with  the  brass  plug  provided. 

OPERATING   CHEST. 

In  this  chest  is  mounted  the  transmitting  and  receiving  apparatus, 
the  diagram  of  which  is  shown  in  figure  80.  To  put  the  chest  in 
operation  for  sending,  connect  the  double  contact  plugs  of  the  leads 
from  the  hand  generator,  field,  antenna,  and  counterpoise  to  the  recep- 
tacles marked  "Gen.,"  "Fid.,"  "A,"  and  "C,"  respectively,  and  the 
four  variable  contact  clips  on  the  leads  from  the  condenser,  spark  gap, 
antenna,  and  hot-wire  ammeter,  to  the  four  points  on  the  flat  spiral, 
as  indicated  on  the  diagram,  making  sure  that  the  counterpoise  clip 
is  at  the  end  of  the  outside  turn.  Set  the  control  switch  at  the 
"sending"  or  lower  position.  Release  the  indicating  needle  of  the 
ammeter  by  turning  the  small  knurled  screw  at  the  left-hand  side 
of  the  upper  binding  post.  When  the  needle  is  free,  adjust  to  zero 
position  on  the  scale  by  means  of  the  small  knurled  screw  at  the 
right  side  of  the  upper  binding  post.  Set  the  variable  spark-gap 
contact  on  the  fifth  plate,  counted  from  the  left  end,  so  as  to  put 
four  gaps  in  circuit.  Start  the  generator,  and  when  the  proper 
speed  is  obtained  the  set  is  ready  for  sending. 

QUENCHED-SPARK   GAP. 

The  spark  gap  used  in  this  set  is  made  up  of  several  copper  disks 
separated  by  mica  washers  about  0.01  inch  thick.  Its  action  is  to 
allow  all  of  the  energy  of  the  closed  oscillating  circuit  to  be  trans- 
ferred to  the  open  or  radiating  circuit  in  a  few  oscillations,  after 
which  the  spark  is  quenched  and  the  circuit  is,  in  effect,  open.  The 
activity  in  the  closed  circuit  having  ceased,  the  open  or  radiating 
circuit  continues  to  oscillate  at  its  own  period,  radiating  waves  of 
its  own  wave  length  without  any  re  transfer  of  energy  to  the  closed 


KADIOTELEGRAPHY.  107 

oscillating  circuit,  which  continues  to  remain  open  until  a  spark  breaks 
down  the  gap  again  at  the  peak  of  the  next  alternation. 

In  order  to  work  at  maximum  efficiency,  the  quenched-spark  gap 
should  be  kept  cool.  It  is  for  this  reason  that  the  plates  are  pro- 
vided with  thin  cooling  flanges,  so  as  to  expose  a  large  surface  to 
the  air,  and  are  blackened,  a  black  body  cooling  more  rapidly  than 
one  highly  polished.  If  the  gaps  become  too  hot  the  antenna  cur- 
rent drops  off  perceptibly,  the  loss  at  times  being  as  high  as  40  per 
cent.  When  thus  greatly  overheated,  or  in  cases  where  the  key  has 
been  closed  continuously  for  a  long  time,  it  is  always  best  to  allow 
the  gaps  to  cool  before  using  again. 

The  gap  should  not  be  taken  apart  to  clean  its  sparking  surfaces 
like  an  ordinary  type  of  open  gap.  In  general  the  more  frequently 
such  a  gap  is  opened  the  more  unsatisfactory  may  be  its  operation. 
The  explanation  is  that  the  repeated  opening  of  the  gaps  introduces 
air  each  time,  and  that  with  free  exposure  to  air  the  sparking  sur- 
faces are  corroded  or  pitted,  but  that  when  kept  air-tight  they  are 
worn  smooth  and  clean  by  the  sparking  action.  Sometimes,  if  there 
is  a  flaw  in  one  of  the  plates  or  if  air  leaks  into  the  gap,  there  will  be 
a  noticeable  drop  in  the  antenna  current,  and  the  note  will  become 
poor.  When  it  is  believed  that  the  trouble  is  confined  to  one  or  two 
gaps  it  is  possible  to  continue  sending  without  dismounting  the 
whole  gap  by  short-circuiting  the  bad  gaps  by  means  of  clips  pro- 
vided for  the  purpose,  in  which  case  as  many  new  gaps  must  be  put 
into  circuit  by  adjusting  the  movable  clip  to  the  right  as  were  cut 
out  by  the  short-circuiting  clips. 

The  gap  should  be  dismounted  only  when  the  trouble  has  been 
located  in  the  gap  and  it  has  been  found  impossible  to  remedy  it  by 
short-circuiting  the  different  gaps  in  use.  The  gap  should  be  dis- 
mounted only  by  an  experienced  man,  who  should  clean  the  surfaces 
by  rubbing  them  face  down  on  fine  emery  cloth  or  paper  on  a  flat 
surface.  It  is  absolutely  necessary  that  both  the  bearing  surface 
and  the  sparking  surface  be  kept  true  and  plane,  as  shown  by  a 
straitedge. 

Great  care  should  be  exercised  in  reassembling  the  gap  to  set  the 
mica  washers  accurately  on  the  annular  surfaces  of  the  disk  and  to 
put  on  enough  tension  on  the  clamping  screws  to  render  all  of  the 
gap  spaces  air-tight. 

TUNING   OF    SENDING   SET. 

The  tuning  of  the  closed  and  open  circuits  to  resonance,  and  the 
determination  of  the  correct  coupling  between  them  are  the  two  most 
important  adjustments  in  a  quenched  spark  transmitter.  In  the  pres- 
ent type  of  directly  coupled  set  with  a  flat  spiral  as  the  oscillation 
transformer,  these  adjustments  can  be  made  either  with  or  without  the 


108  RADIOTELEGRAPH  Y. 

help  of  a  wave  meter.  If  made  without  the  meter,  the  adjustments 
should  satisfy  the  following  tests:  (1)  The  number  of  turns  in  the 
closed  circuit  should  be  chosen  to  give  the  desired  wave  length;  (2) 
the  antenna  hot-wire  ammeter  should  show  the  maximum  reading; 
(3)  the  note  should  be  clear  and  characteristic  of  500  cycles.  These 
adjustments  are  in  general  dependent  on  each  other,  an  incorrect 
change  in  one  seriously  affecting  all  the  others.  As  shown  in  the 
table,  the  position  of  the  clips  in  the  closed  circuit  determines  the 
proper  turns  in  the  spiral  to  give  the  desired  wave  lengths. 

The  number  of  turns  in  the  open  or  radiating  circuit  necessary  to 
bring  it  into  resonance  with  the  closed  circuit  must  be  found  by 
trial,  although  the  numbers  shown  in  the  table  below  are  approxi- 
mately correct  and  should  be  used  in  beginning  to  make  the  adjust- 
ments. While  the  open  circuit  is  being  tuned,  the  character  of  the 
note  can  be  determined  by  listening  in  the  telephones  of  the  receiving 
set,  which  although  disconnected  at  the  control  switch  are  sufficiently 
energized  by  the  transmitter  so  that  the  note  will  be  heard  nearly  the 
same  as  by  the  receiving  operator  at  the  distant  station.  It  will  be 
found  that  the  character  of  the  note  will  be  changed  as  changes  are 
made  in  the  coupling  of  the  circuits  and  in  the  tuning  of  the  open 
circuit.  If  the  circuits  are  in  resonance  and  the  coupling  is  correct, 
the  antenna  hot-wire  ammeter  should  read  not  less  than  2.2  amperes 
and  may  read  as  high  as  3.0  amperes.  If  the  ammeter  reading  is  low, 
then  either  the  coupling  is  too  loose  or  the  circuits  are  not  in  reso- 
nance, and  one  or  both  of  the  clips  must  be  moved  so  as  to  get  the 
highest  possible  reading  consistent  with  a  clear  note.  It  is  impossible 
to  indicate  which  clip  is  to  be  moved  or  in  which  direction,  but  if 
possible  the  counterpoise  clip  should  be  kept  on  the  outside  turn. 
It  is  possible  that  even  when  both  the  coupling  and  the  tuning  of  the 
circuits  are  correct,  the  note  may  not  be  clear.  It  may  be  of  high 
pitch,  but  with  a  hissing  sound,  in  which  case  the  closed  circuit  con- 
denser is  being  charged  and  discharged  more  than  once  per  alter- 
nation, and  the  remedy  is  to  increase  the  number  of  gaps  until  the 
note  clears.  Similarly  the  note  may  be  of  low  pitch  and  ragged,  in 
which  case  the  condenser  is  being  discharged  irregularly  at  every 
second  or  third  alternation.  If  the  note  is  of  low  pitch  and  clear 
the  condenser  is  being  discharged  regularly  every  second  or  third 
alternation.  In  both  cases  the  remedy  is  to  decrease  the  number  of 
gaps  until  the  note  clears.  The  final  adjustment  must  be  such  as  to 
give  the  maximum  antenna  current  consistent  with  the  clear,  high- 
pitched  note  characteristic  of  500  cycles. 

Although  there  is  no  direct  test  that  can  be  applied,  except  with 
a  wave  meter,  to  determine  if  a  single  sharply  defined  wave  length  is 
being  radiated,  yet  in  general  this  will  be  the  case  if  these  conditions 
are  fulfilled. 


RADIOTELEGRAPH  Y . 


109 


In  a  few  cases  a  wave  meter  may  be  available  in  making  the  adjust- 
ments and  in  this  case  the  current  and  voltage  from  the  hand-driven 
generator  may  not  be  steady  enough  to  permit  .of  its  use.  Under 
these  conditions  either  the  one-eighth  or  one-fourth  kilowatt  motor- 
generator  or  the  engine-driven  one-fourth  kilowatt  generator  sup- 
plied by  the  Signal  Corps  should  be  used,  if  possible,  as  the  source  of 
the  500-cycle  current.  When  the  motor  generator  set  is  used  the 
A.  C.  armature  and  the  D.  C.  motor  should  be  protected  from  "  kick- 
backs''  due  to  the  use  of  the  sending  key  in  the  alternator  fields.  For 
this  purpose  two  high-resistance  carbon  rods  mounted  on  suitable 
bases  have  been  provided,  to  be  connected  as  follows :  The  end  termi- 
nals of  one  rod  to  the  two  A.  C.  leads  close  to  the  machine;  the  end 
terminals  of  the  other  rod  to  the  two  main  line  D.  C.  leads  close  to 
the  machine,  and  the  middle  points  of  both  rods  to  be  connected  to- 
gether and  this  common  point  to  be  grounded  on  the  frame  of  the 
machine. 

A  table  of  the  wave  lengths  of  the  open  circuit  can  be  made  by 
connecting  the  counterpoise  to  one  terminal  of  a  zinc  gap,  the  other 
terminal  of  which  should  be  connected  to  the  end  of  the  outside  turn 
of  the  flat  spiral.  The  two  transformer  terminals  should  be  con- 
nected to  the  two  terminals  of  the  gap.  The  antenna  contact  should 
be  put  on  the  different  inside  turns  and  the  wave  length  measured 
each  time,  giving  such  a  curve  as  that  shown  in  the  table  below,  which 
is  approximately  accurate  for  the  standard  antenna: 

OPEN-CIRCUIT   TUNING. 


Wave  length. 

Antenna. 

Counterpoise. 

Meters. 
300 
325 
350 
375 
400 
425 

Turn 
26j 
24i 
22j 
20 
18 
16 

No. 

Turn  No. 
30 
30 
30 
30 
30 
30 

Turns  to  be  counted  from  the  inside  turn  outward. 

Set  the  closed  and  the  open-circuit  contacts  to  give  the  desired 
wave  length  and  note  the  reading  of  the  antenna  hot-wire  ammeter. 
At  the  same  time  test  with  the  wave  meter  near  the  antenna  or 
counterpoise  wires,  but  not  near  the  spiral,  to  see  if  a  single  wave 
length  is  being  radiated.  It  may  be  found  that  the  antenna  ammeter 
reading  is  low,  less  than  2.2  amperes,  and  that  the  wave  meter  shows 
only  one  wave  length,  in  which  case  the  coupling  must  be  increased. 
This  may  be  done  either  by  moving  the  closed  circuit  turns  in  use 
outward  or  the  open  circuit  turns  in  use  inward  as  a  whole,  but  it 
must  be  remembered  that  slight  changes  in  tuning  in  both  circuits 


110 


RADIOTELEGR  APH  Y. 


will  be  necessary  as  the  wave  length  of  each  has  been  slightly  changed 
because  the  inductance  changes  as  the  diameter  of  the  turns  changes 
even  though  the  same  number  of  turns  is  in  circuit,  It  may  be  pos- 
sible to  increase  the  coupling  still  further  and  to  get  a  larger  antenna 
current  and  still  have  a  single  wave  length,  in  which  case  corresponding 
changes  may  have  to  be  made  in  the  tuning  of  the  two  circuits.  While 
these  changes  are  being  made  the  character  of  the  note  should  be  de- 
termined either  in  the  telephones  of  the  receiving  set  or  of  the  wave 
meter,  to  see  if  it  is  clear  and  of  500-cycle  pitch.  If  it  is  not,  then 
changes  in  the  number  of  gaps  must  be  made  as  described  above.  The 
final  adjustments  must  be  such  as  to  give  the  maximum  current  in  the 
antenna  hot-wire  ammeter  with  a  single  wave  length  of  the  desired 
value  in  the  wave  meter  and  with  a  clear  500-cycle  note  in  the  tele- 
phones. After  the  adjustments  have  been  completed  at  this  wave 
length  they  should  be  repeated  at  the  lengths  within  the  range  of 
the  spiral  and  the  results  tabulated  as  below. 


Wave  length. 

Closed-circuit 
clips. 

Open-circuit 
clips. 

Meiers. 
300 
325 
350 
375 
400 
425 

Turns. 
8  and  12* 
8  and  13£ 
8  and  13| 
8  and  14| 
8  and  14| 
8  and  15  J 

2& 
24^ 
22' 
20> 
18 
16 

*urns. 
and  30 
and  30 
and  30 
and  30 
and  30 
and  30 

Turns  counted  from  the  inside  turn  outward. 

Although  a  transmitting  set  using  the  flat  spiral  oscillation  trans- 
former is  not  as  easily  tuned  as  some  other  types,  yet  when  the  ad- 
justments have  once  been  made  and  tabulated  it  is  practically  as 
efficient  as  other  types.  It  has  the  advantage  of  being  one  of  the 
simplest,  most  rugged,  and  compact  forms  which  can  be  installed  in  a 
field  set. 

RECEIVING   SET. 

The  receiving  set  consists  of  an  inductively  connected  transformer, 
perikon,  or  other  similar  detector,  high-resistance  telephones,  etc., 
provided  with  the  necessary  switches  for  tuning  to  different  wave 
lengths.  The  primary  circuit  includes  the  antenna,  coil,  series  con- 
denser (or  not,  as  may  be  needed),  and  counterpoise.  The  antenna 
is  connected  to  the  point  A  and  thence  to  the  primary  coil  through 
switches  which  put  into  circuit  a  variable  number  of  turns,  steps  of 
10  turns  being  inserted  by  one  dial  switch  and  single  turns  by  the 
other.  The  total  number  of  primary  turns  is  thus  the  sum  of  the 
numbers  on  the  two  dials  indicated  by  the  two  switch  arms,  which 
can  be  varied  by  single  turns  from  one  to  the  whole  number  in  the 


EADIOTELEGEAPHY. 


Ill 


coil.  For  wave  lengths  shorter  than  the  fundamental  wave  length 
of  the  antenna,  a  fixed  condenser  can  be  inserted  in  series  with  the 
coil  by  throwing  the  short-circuiting  switch  to  the  position  "In," 


FIELD  RADIO  PACK  SET,  MODEL  1914 
DIAGRAM 


FIG.  80. 


as  shown  in  figure  80.  For  the  longer  wave  lengths  the  switch  is 
thrown  to  the  other  position,  short-circuiting  the  condenser,  and  thus 
leaving  "only  the  coil  in  circuit.  The  secondary  circuit  includes  the 
secondary  coil,  detector,  and  the  stopping  condenser  shunting  the 


112 


RADIOTELEGRAPHY. 


telephones.  The  coil  is  variable  only  by  sections,  marked  100,  200, 
etc.,  the  smaller  numbers  to  be  used  at  the  shorter  wave  lengths  and 
the  larger  ones  at  the  longer  wave  lengths.  The  position  of  the  sec- 
ondary coil  within  the  primary — that  is,  the  coupling — is  variable, 
and  for  the  sake  of  convenience  a  scale  is  provided  so  as  to  be  able  to 
note  the  different  adjustments.  The  coupling  is  closest  when  the 
secondary  is  inside  the  primary,  in  which  case  the  scale  reading  is  0, 
and  vice  versa,  the  coupling  is  loosest  when  the  secondary  is  drawn 
outside  the  primary  and  the  scale  reading  is  40. 

Primary  condenser  short-circuited. 


1 

Wave 
length  (in 
meters). 

Primary 
turns. 

Secondary 
turns. 

Coupling 
scale. 

300 

24 

200 

20 

400 

30 

200 

20 

500                   38 

300 

20 

600 

46 

300 

20 

700 

56 

400 

25 

800 

65 

400 

30 

900 

76 

400 

30 

1,000 

91 

400 

25 

1,100 

107 

400 

25 

1,200 

125 

400 

30 

1,300 

144 

400 

25 

1,400 

162 

400 

25 

Etc. 

Etc. 

Etc. 

Etc. 

Primary  condenser  in  series. 
[Switch  on  "In"  contact.] 


Wave 
length  (in 
meters). 

Primary 
turns. 

Secondary 
turns. 

Coupling 
scale. 

200 
300 
400 
500 
600 
700 
800 
Etc. 

18 
26 
36 
47 
60 
74 
88 
Etc. 

100 
200 
200 
300 
300 
400 
400 
Etc. 

20 
20 
20 
20 
20 
25 
30 
Etc. 

TUNING    OF   THE    RECEIVING    SET. 

First,  the  detector  must  be  adjusted  to  a  sensitive  point  by  means 
of  the  test  buzzer,  the  note  of  which  should  be  clearly  heard  in  the 
receiving  telephones  when  it  is  held  near  the  antenna  or  counterpoise 
wires  or  the  coil  windings.  When  the  wave  length  of  the  sending 
station  is  known,  the  number  of  turns  in  the  primary  and  secondary 
coils  and  the  coupling  should  be  set  according  to  the  values  in  the 
above  table,  which  will  be  approximately  correct  for  all  sets  using 
the  standard  antenna.  When  the  wave  length  is  unknown,  then 
signals  can  be  found  only  by  repeated  trials  of  different  combinations 
of  turns  and  couplings,  in  which,  however,  consistent  sets  of  values 


RADIOTELEGRAPH  Y.  113 

may  be  taken  from  the  table.  When  once  the  signals  have  been  heard 
such  further  adjustments  of  primary  and  secondary  turns  and  cou- 
pling should  be  made  as  will  give  the  maximum  sound  in  the  tele- 
phones. In  general  it  will  be  found  that  when  there  is  interference 
or  static  troubles  the  sharpest  tuning  and  the  best  protection  from 
interference  will  be  obtained  when  the  loosest  coupling  is  used;  that 
is,  when  the  secondary  is  pulled  out  as  far  as  possible  and  still  hear 
the  desired  station.  If  the  signals  are  of  short  wave  lengths,  the 
series  condenser  should  be  inserted  in  the  antenna  circuit  by  throw- 
ing the  condenser  switch  to  the  position  "In,"  in  which  case  the  wave 
lengths  will  be  as  given  in  the  above  table.  It  will  be  noticed  that 
for  some  wave  lengths  there  are  two  different  possible  combina- 
tions in  the  primary  circuit,  either  without  a  condenser  and  a  few 
primary  turns  or  with  a  condenser  and  more  primary  turns.  It  is 
impossible  to  tell  which  combination  is  the  better  without  actual  trial. 
In  general  the  best  coupling  between  the  circuits  will  vary  with  the 
damping  of  the  transmitting  station,  close  coupling  being  possible 
with  highly  damped  transmitters,  and  loose  coupling  necessary  with 
feebly  damped  transmitters. 

In  changing  the  coupling  between  the  two  circuits  by  means  of  the 
handle  on  the  secondary  coil  care  must  be  taken  to  see  that  the  con- 
tacts on  the  various  studs  are  not  loosened,  as  otherwise  the  signals 
may  be  lost  entirely  or  the  tuning  made  much  broader  on  account 
of  high  resistance  that  may  be  introduced  at  these  contacts. 

If  the  receiver  is  used  with  the  standard  antenna  and  signals 
are  being  received  from  an  unknown  station,  the  table  of  wave  length 
can  be  used  to  determine  approximately  the  wave  length  of  the  un- 
known station. 

SHELTER    TENT. 

This  tent  is  similar  in  dimensions  and  construction  to  the  standard 
"common"  wall  tent  issued  by  the  Quartermaster's  Department,  but 
is  made  of  lighter  material  and  is  not  provided  with  ridge  pole  or 
uprights.  In  erecting  the  tent  the  extra  sections  furnished  with  the 
mast  should  be  used  as  the  ridge  pole  and  uprights  as  follows:  One 
hollow  section,  one  plug,  and  one  extension  piece  for  the  ridge,  and 
one  section,  one  extension  piece  with  spike  for  each  upright.  The 
method  of  erection  is  illustrated  in  figure  81. 

INSULATING    DEVICE. 

A  device  is  provided  for  use  in  insulating  the  aerial  when  the 
shelter  tent  is  used  in  damp  weather,  consisting  of  a  square  piece  of 
sheet  rubber  with  small  marginal  holes  for  lacing  into  the  ventilator 
at  either  end  of  the  tent,  and  a  tube  attached  to  the  center  for  ad- 

92448—15 8 


114 


KADIOTELEGKAPHY. 


mitting  the  aerial  lead.  When  in  use,  sufficient  slack  should  be  left 
in  the  aerial  lead  to  form  a  drip  loop  outside  of  the  tent,  and  if 
found  necessary  a  piece  of  heavy  insulated  wire  can  be  used  as  a 
leading-in  wire. 


PACKING. 


The  set  is  normally  packed  on  three  mules,  but  in  emergency  may 
be  packed  on  two.     In  normal  packing  the  first  mule  carries  the  gen- 


SPIKE 


SPIKE- 


RIDGE  POLE:  EXTENSION  PIECE 


TENT  POLE  EXTENSION   PIECE 


TENT  POLE    EXTENSION  PIECE 


FlG.  81. 

erator  and  six  sections  of  the  mast.  The  second  mule  carries  the 
operating  chest,  four  sections  of  the  mast,  antenna,  counterpoise; 
accessories,  bag,  etc.  The  third  mule  carries  the  tent,  with  tent  pins 
and  extension  pieces  folded  inside,  four  sections  of  the  mast,  flag  kit, 
lanterns,  etc.  In  emergency  packing  with  two  mules,  the  first  mule 
carries  the  generator  and  10  sections  of  the  mast,  and  the  second  the 
operating  chest,  four  sections  of  the  mast,  antenna,  counterpoise,  and 
tent.  Figures  82  and  83  show  the  present  methods  of  packing. 


EADIOTELEGKAPHY. 


115 


The  1915  model  pack  set  is  similar  to  the  previous  types  and  in 
general  the  same  instructions,  etc.,  apply  to  it  as  to  the  others.  The 
essential  differences  are  in  the  transmitting  oscillation  transformer 
and  the  receiving  set,  a  brief  description  of  which  will  be  given. 


OSCILLATION    TRANSFORMER. 


The  oscillation  transformer  consists  of  two  open  spirals  inductively 
coupled  and  a  third  spiral  which  is  to  be  used  as  an  antenna  induc- 
tance for  obtaining  longer  wave  lengths.  This  inductance  is  inserted 
between  the  oscillation  transformer  and  the  antenna  by  transferring 


116 


RADIOTELEGRAPHY. 


the  long  flexible  lead  from  the  open  circuit  spiral  to  the  inductance 
which  is  in  turn  connected  to  the  oscillation  transformer  by  a  short 
flexible  connection.  Care  must  be  taken  to  see  that  these  added  turns 
do  not  oppose  the  turns  of  the  oscillation  transformer,  that  is,  the 
inside  turns  of  one  should  be  connected  to  the  inside  turns  of  the  other. 


The  open  and  closed  circuits  of  the  oscillation  transformer  are 
electrically  joined  together  at  their  base,  to  which  is  directly  con- 
nected the  counterpoise  through  control  switch  and  ammeter.  This 
method  of  construction  reduces  the'number  of  movable  contacts  from 


BADIOTELEGBAPHY.  117 

four  to  two  and  also  has  the  advantage  that  the  outside  metal  rings 
may  be  handled  without  danger  of  shock. 

Ordinarily  the  antenna  inductance  will  not  be  in  circuit  except  for 
long  wave  lengths  and  when  not  so  used  it  should  be  swung  out  a  few 
inches  from  the  lid  of  the  chest.  The  tuning  of  the  closed  and  open 
circuits  to  resonance,  and  the  determination  of  the  correct  coupling 
between  them,  are  the  two  important  adjustments  of  an  inductively 
coupled  quenched  spark  transmitter.  The  adjustments  should  be 
made  to  satisfy  the  following  tests : 

(1)  The  short  flexible  connection  from  the  rear  binding  post  of 
condenser  should  be  attached  by  the  plug  to  the  small  angle  piece 
extending  out  at  right  angle  from  the  base  of  the  oscillation  trans- 
former; (2)  the  longer  connection  at  the  opposite  end  of  the  condenser 
should  be  attached  to  the  number  of  turns  in  the  closed  circuit  spiral 
which  will  give  the  desired  wave  length;  (3)  the  open  circuit  spiral 
should  be  connected  to  the  antenna  by  the  flexible  cord  connection 
from  the  control  switch,  picking  off  the  right  number  of  turns  which 
will  bring  the  open  and  closed  circuits  into  resonance.  The  right 
number  of  turns  must  be  found  by  trial. 

In  tuning  these  circuits  the  two  spirals  should  be  swung  apart 
from  8  to  10  inches.  After  the  two  circuits  have  been  brought  into 
resonance,  as  indicated  by  the  greatest  deflection  of  the  hot  wire 
ammeter,  the  coupling  of  the  two  circuits  should  be  increased  or  made 
tighter  by  gradually  swinging  the  spirals  closer  together  until  the 
ammeter  deflection  just  begins  to  decrease.  If  a  wave  meter  is 
available  or  a  distant  station  assists  in  the  test,  a  single  wave  length 
or  "hump"  should  be  transmitted  and  a  clear  note  obtained.  Care 
should  be  taken  not  to  have  too  close  a  coupling. 

When  the  standard  closed-circuit  condenser  is  used  with  this 
oscillation  transformer,  the  wave  lengths  are  very  approximately 
given  in  the  following  table : 

Wave  lengths  of  primary  or  closed  oscillating  circuits. 

Number  of 

IT-         i        ,1     •  primary 

\\ave  length,  in  meters:  turns. 

300 2 

400 34 

500 5 

600 6J 

700 84 

800 10 

1,000 15 

13  200 22 

NOTE. — Turns  counted  from  the  outside  turn  inward. 


118  RADIOTELEGRAPH  Y. 

RECEIVING    SET. 

In  the  earlier  sets,  types  A  and  B,  the  two  circuits  were  magneti- 
cally coupled,  that  is,  the  current  in  the  primary  (open  or  antenna) 
circuit  induced  currents  in  the  secondary  (closed  or  detector)  circuit 
by  means  of  magnetic  lines  which  passed  from  the  primary  coil 
through  the  turns  of  the  secondary  coil.  In  the  present  set  the  two 
circuits  are  statically  coupled,  that  is,  the  current  in  the  primary 
circuit  induces  current  in  the  secondary  circuit  by  means  of  static 
lines  in  two  coupling  condensers  connected  in  the  leads  between  the 
circuits.  The  transfer  of  the  energy  for  the  operation  of  the  detector 
and  telephones  from  the  primary  to  the  secondary  circuit  is  as 
efficient  in  this  type  of  connection  as  in  the  other.  By  choice  of 
suitable  values  of  the  coupling  condensers  no  movement  of  the  coils  or 
changes  in  coupling  is  necessary  for  the  reception  of  any  wave  lengths 
within  the  range  of  the  set,  as  in  the  case  is  the  former  sets.  This 
reduces  the  number  of  adjustments  for  tuning  from  4  to  3,  and  at  the 
same  time  the  set  is  much  more  rugged,  as  there  are  no  moving  parts. 
The  values  of  the  coupling  condenser  have  also  been  so  chosen  as  to 
make  the  set  much  more  selective  than  the  others,  that  is,  it  can 
receive  signals  from  a  station  on  one  wave  length  and  cut  out  signals 
from  another  station  on  a  different  wave  length  more  completely 
than  before.  In  addition  to  the  above  advantages,  the*set  as  a  whole 
has  been  found  to  be  more  efficient  than  the  previous  types. 

The  type  C  receiving  set  consists  of  two  statically  coupled  circuits, 
high-resistance  telephones,  stopping  condenser,  fine  wire-crystal  de- 
tector, switch  for  short  and  long  wave  lengths,  three  dial  switches 
for  tuning,  etc.  The  circuits  are  shown  diagrammatically  in  the 
attached  prints. 

The  primary  circuit  consists  of:  (1)  The  antenna,  which  when 
the  control  switch  in  the  cover  of  the  chest  is  thrown  to  the  "Receive" 
position  is  connected  by  a  double  plug  with  flexible  wires  to  the 
binding  post  on  the  set  marked  "A";  (2)  two  primary  coils  in  series, 
one  large  and  the  other  small,  the  number  of  turns  in  both  of  which 
is  variable  by  means  of  the  two  dial  switches  marked  "Primary"; 
on  each  coil  there  are  contacts,  0  to  24,  for  tuning  to  different  wave 
lengths,  the  dial  nearest  to  the  binding  post  "A"  being  connected 
to  the  large  primary  for  large  changes  in  wave  length  and  the  other 
to  the  small  one  for  small  changes  and  fine  tuning;  (3)  counterpoise 
which  is  connected  to  the  binding  post  marked  "C"  through  the 
double  plug  and  control  switch.  There  is  no  series  condenser  in  the 
antenna  circuit  for  the  reception  of  wave  lengths  shorter  than  the 
fundamental  wave  length  of  the  antenna,  as  in  types  A  and  B,  as  it 
has  been  found  not  to  be  generally  useful. 


RADIOTELEGEAPHY. 


119 


V 

/                   SHORT  WAVE  LENGTHS 

\ 

/Antenna 

300  TO  700  METERS 

8 

b 

O 

C51 

\\ 

•1 

8 

o 

>             Fixed 
coup/ing       C 

M      1 

'§3       Detector  Q 

^            condensers 

t?<o 

6 

0 

Telephone  cond. 

| 

0*^ 

o 

nn 

l^^J 

•  y^ 

o 

/*^X^                ^^**v 

0, 

0 
0 

©  © 

"^Ground 

Telephone 

.           /                 LONG  WAVE  LENGTHS 

N| 

/   7                   500  TO  1500  METERS 
/Antenna 

^ 

^- 

0 

c 

O 

.6 

^  

ii 

0 

Q^ 

o 

^              Fixed                                •    ; 
coupling               Detector  •£ 

n®l 

P            ^*§ 

o              condensers 

P—  VP)-^ 

0 

^v 

0 

^     p; 

n 

8<           "             "™"i 

1 

n                  •—  i 

^^    t^,^ 

•*m 

•s 
" 

WGround 

: 

TYPE  C  RECEIVING  SET 

DIAGRAMATIC  CIRCUIT 


120 


KADIOTELEGRAPHY. 


KADIOTELEGRAPHY.  121 

When  comparatively  short  wave  lengths  are  to  be  received,  as 
from  300  to  700  meters,  the  double-pole  double-throw  switch  on  top 
of  the  set  should  be  thrown  to  the  position  marked  "Short";  this 
makes  no  changes  in  the  primary  circuit  but  connects  into  circuit: 
(1)  The  secondary  coil  with  the  dial  switch  marked  " Secondary'' 
with  contacts  0  to  24  for  tuning  to  different  wave  lengths;  (2)  detector 
and  telephones. 

When  longer  wave  lengths  are  to  be  received,  as  from  500  to  1 ,500 
meters,  the  D-P  D-T  switch  should  be  thrown  to  the  "Long"  posi- 
tion; this  makes  no  changes  in  the  primary  circuit  but  disconnects 
the  secondary  coil,  which  in  this  set  is  most  useful  only  at  short 
wave  lengths,  and  connects  the  circuits  as  shown  in  the  second  print. 
As  the  secondary  coil  is  not  in  circuit,  only  the  two  primary  dials  are 
effective  in  tuning. 

Short  wave  signals  should  be  picked  up  by  adjustments  of  the  large 
primary  and  the  secondary  dials  and  fine  adjustments  made  later 
on  the  small  primary  dial. 

Long  wave  signals  should  be  picked  up  only  by  adjustment  of  the 
large  primary  dial  and  fine  adjustments  made  later  only  on  the  small 
primary  dial. 

.       TRACTOR    SETS. 

The  Signal  Corps  has  designed  and  built  two  sizes  of  automobile 
radio  sets  or  tractor  sets,  as  they  are  called — (a)  a  "divisional"  tractor 
of  1  k.  w.  size;  (6)  an  "Army"  tractor  of  2  k.  w.  size. 

The  1  k.  w.  set,  complete  with  supplies  and  detachment  of  seven 
men,  weighs  about  6,700  pounds,  and  on  an  average  road  is  capable 
of  making  a  speed  of  from  20  to  25  miles  per  hour.  It  carries  a  60- 
foot  sectional  mast  which  can  be  raised  in  a  few  minutes  by  means  of 
guides  on  the  roof  of  the  tractor.  The  antenna  is  of  the  umbrella 
type,  with  16  radiating  wires  each  75  feet  long.  The  counterpoise  is 
likewise  of  the  umbrella  type,  laid  on  the  ground  with  8  wires,  each 
75  feet  long.  The  transmitting  set  is  of  the  quenched-spark  type, 
with  inductively  coupled  circuits  adjusted  to  radiate  waves  of  600, 
800,  1,000,  and  1,200  meters.  The  receiving  set  is  of  the  statically 
coupled  type  similar  to  that  in  use  in  the  1915  radio  pack  sets,  but 
of  larger  size  and  capable  of  reception  of  much  longer  wave  lengths. 

The  2  k.  w.  set  complete,  with  supplies  and  detachment  of  8  men, 
weighs  about  9,000  pounds,  and  on  an  average  road  is  capable  of 
making  a  speed  of  at  least  15  miles  per  hour.  It  carries  an  80-foot 
sectional  mast  which  is  raised  in  a  manner  similar  to  that  in  the  1 
k.  w.  set.  The  transmitting  and  receiving  sets  are  likewise  similar  to 
those  in  the  previous  set,  but  capable  of  using  much  longer  wave 
lengths. 


APPENDIX. 


DAMPING— LOGARITHMIC   DECREMENT. 

The  oscillations  in  a  wave  train  in  a  single  circuit  of  coil  and  con- 
denser die  down  to  zero,  as  shown  in  figure  13.  Other  things  being 
equal,  the  higher  the  resistance  the  more  rapid  is  the  decrease  in 
amplitude  of  each  successive  oscillation;  that  is,  the  higher  the  damp- 
ing; and,  vice  versa,  the  lower  the  resistance  the  less  rapid  is  this 
decrease  and  the  smaller  the  damping.  In  every  circuit  in  which  the 
resistance  is  constant  any  amplitude  in  the  train  is  a  constant  frac- 
tional part  of  the  preceding  amplitude. 

It  is  possible  to  compare  the  relative  amplitudes  of  the  oscillations 
in  this  way  and  thus  to  indicate  the  rate  at  which  they  decrease.  For 
purely  theoretical  reasons,  however,  the  measure  of  the  damping  has 
been  taken  as  the  natural  logarithm,  sometimes  called  naperian  or 
hyperbolic  logarithm,  of  the  ratio  of  two  successive  amplitudes  in  the 
same  direction.  The  symbol  for  this  expression  which  is  constant 

for  a  wave  train  is  generally  written  d.     Thus  log£^  =  d,  where  It 

-^2 

is  the  amplitude  of  any  oscillation  as  at  B,  in  figure  13,  I2  the 
amplitude  of  the  next  oscillation  in  the  same  direction  as  at  F;  and  d 
is  the  logarithmic  decrement,  or  simply  decrement,  the  significance 
of  which  term  will  be  given  later.  Although  the  amplitudes  are  both 
positive,  the  same  formula  applies  when  both  amplitudes  are  nega- 
tive. In  both  cases  the  amplitudes  are  one  complete  oscillation  apart 
and  hence  the  decrement  when  so  measured  is  called  the  decrement 
per  complete  oscillation.  In  a  few  cases  the  logarithm  of  the  ratio 
of  two  successive  amplitudes  in  opposite  directions  is  used,  in  which 
case  the  decrement  is  per  half  oscillation,  and  numerically  it  is  one- 
half  the  decrement  per  complete  oscillation.  The  decrement  per 
complete  oscillation  is  always  used  in  practical  work  in  this  country. 

Natural  logarithms  are  indicated  by  writing  the  letter  £  as  a  sub- 
script; thus,  log£2  where  e  is  the  base  of  the  natural  system  of  log- 
arithms, s  being  the  number  2.71828.  (In  some  cases  in  books 
on  pure  mathematics  the  subscript  may  be  omitted.)  No  subscript 
is  used  with  the  common  or  ordinary  logarithms,  the  base  of  which 
is  10. 

Tables  of  natural  logarithms  are  sometimes  used,  although  not 
convenient  for  most  computations.  The  natural  logarithm  can,  how- 
122 


RADIOTELEGRAPHY. 


123 


ever,  be  found  by  multiplying  the  common  logarithms  by  2.3026; 
thus,  log  3.000  =  0.4771,  Ioge3.000  =  0.4771  X  2.  3026  =  1.099,  as  would 
be  found  directly  in  a  table  of  natural  logarithms. 

The  expression  £  =  log£  ^    can  be    written   ^  =  logJ1-logJ2,    the 


logarithm  of  the  fraction  being  the  logarithm  of  the  numerator  minus 
the  logarithm  of  the  denominator.  The  expression  can  also  be 
written  log£  I1  —  d  =  logc  I2,  in  which  form  it  is  seen  that  as  d  is  con- 
stant for  any  one  wave  train,  the  natural  logarithm  of  the  amplitude 
of  any  oscillation  can  be  obtained  by  subtracting  the  constant  quan- 
tity d  from  the  natural  logarithm  of  the  next  preceding  amplitude 
in  the  same  direction.  The  term  logarithmic  decrement,  or  simply 
decrement,  as  mentioned  above,  thus  receives  its  name  from  the  fact 
that  it  is  the  constant  quantity  by  which  the  logarithm  of  any  ampli- 
tude must  be  decreased  so  as  to  give  the  logarithm  of  the  next  ampli- 
tude in  the  same  direction. 

A  simple  illustration  of  the  decrement  is  given  in  the  table  below, 
where  in  the  first  column  are  given  the  numerical  values  of  the 
successive  amplitudes  in  a  wave  train,  beginning  for  convenience 
with  a  value  of  10.  Each  amplitude  is  a  constant  fractional  part, 
0.818  approximately,  of  the  preceding;  in  the  second  column  is  the 
common  logarithm  of  the  amplitudes;  in  the  third  column  the 
natural  logarithm;  and  in  the  fourth  column  the  decrement 
^  =  log£I1-log£I2. 


Ampli- 
tudes. 

Common 
logarithm 
of 
amplitudes. 

Natural 
logarithms. 

Decrement 
or 

§ 

10.00 
8.18 
6.70 
5.49 
4.50 

1.0000 
0.9128 
0.  8261 
0.  7396 
0.6532 

2.  3026 
2.  1026 
1.9026 
1.  7026 
1.5026 

0.200 
0.200 
0.200 
0.200 

From  this  table  it  is  seen  that  the  decrement  of  this  wave  train 
is  0.20,  which  is  very  closely  represented  in  figure  14.  Similarly  in 
figure  13  the  decrement  is  0.4,  and  in  figure  15  in  the  case  of  un- 
damped oscillations  it  is  zero. 

MEASUREMENT    OF   LOGARITHMIC   DECREMENT. 

The  subject  of  damping  and  its  measurement  in  terms  of  the 
logarithmic  decrement  is  one  of  the  most  technical  parts  of  the 
subject  of  radio  telegraphy  so  that  only  a  brief  outline  of  the  simplest 
cases  can  be  given  here. 

The  logarithmic  decrement  can  be  measured  either  directly  by  a 
decremeter  which  is  a  modified  form  of  a  wave  meter,  or  by  a  wave 
meter  if  it  is  provided  with  a  suitable  means  of  indicating  resonance. 


124  RADIOTELEGRAPH  Y. 

When  a  wave  meter  is  adjusted  to  resonance  with  a  circuit  in 
which  oscillations  are  taking  place  it  will  be  found  that  the  larger  the 
resistance  in  the  circuit  the  broader  will  be  the  tuning  in  the  wave 
meter — i.  e.,  the  greater  will  be  the  change  that  must  be  made  in 
the, wave-meter  condenser  to  make  any  decrease  in  the  wave-meter 
current  from  the  value  at  resonance.  Similarly  the  larger  the  resis- 
tance in  the  wave-meter  circuit  the  broader  will  be  the  tuning.  On 
the  other  hand  the  smaller  the  resistances  in  both  the  circuit  and 
the  wave  meter  the  sharper  will  be  the  tuning.  As  has  been  previ- 
ously stated  on  page  120,  the  less  the  resistance  in  the  circuit  the  less 
will  be  the  damping,  and  hence  the  smaller  the  logarithmic  decrement. 
Thus  it  is  seen,  in  a  general  way,  that  there  is  a  relation  between  the 
shape  and  breadth  of  the  resonance  curve  and  the  decrement  of  the 
circuit  under  measurement. 

It  has  been  shown  by  theory  that  if  the  resonance  curve  is  taken 
by  a  wave  meter  under  certain  standard  conditions,  a  simple  formula 
can  be  used  to  find  the  logarithmic  decrement  of  a  circuit.  For  this 
purpose  the  wave  meter  should  have  a  variable  condenser  with  a 
suitable  scale,  graduated  from  0  to  180  or  0  to  90  degrees,  with 
which  there  is  furnished  a  calibration  curve  of  the  capacity  of 
the  condenser,  the  wave  length  indicated  by  the  meter;  and  a 
hot-wire  wattmeter  with  a  suitable  scale,  connected  as  shown  in 
figure  48.  The  wattmeter  indicates  the  value  I2R  in  fractions  of  a 
watt,  where  I2  is  the  square  of  the  current  flowing  in  the  wattmeter 
wire  and  B,  is  its  high-frequency  resistance.  This  wire  is  generally 
made  of  a  special  alloy  which  does  not  change  its  resistance  appre- 
ciably with  heating  and  hence  the  product  I2R,  that  is,  the  watts 
on  the  scale  of  the  wattmeter,  can  be  taken  as  relative  values  of  I2, 
and  of  the  squares  of  the  currents  in  the  wave-meter  circuit.  Thus 
if  for  two  different  currents  the  wattmeter  scale  deflections  are 
0.35  X  1/10  watts  =0.035  watts  and  0.0175  watts,  the  relative  values 
of  I2  are  1  and  J. 

The  logarithmic  decrement  of  a  circuit  can  be  measured  as  follows: 
Couple  the  wave  meter  loosely  with  the  circuit  and  adjust  the  vari- 
able condenser  until  resonance  is  obtained.  Adjust  the  coupling 
slightly  until  the  wattmeter  needle  is  on  some  convenient  scale  divi- 
sion at  or  near  full  scale  reading.  Note  this  wattmeter  reading,  IE2 
and  the  condenser  capacity,  CR.  Without  changing  the  coupling  ad- 
just the  variable  condenser  toward  the  zero  end  of  its  scale;  that  is, 
for  smaller  values  of  capacity  and  for  shorter  wave  lengths  than  at 
resonance  until  the  wattmeter  reading  is  reduced  to  one-half  of  its 

I  2 
value  at  resonance.     Note  this  reading,  -^-  =  1^,  and  the  condenser 

£ 

capacity,  Cx.     Similarly,  without  changing  the  coupling,  adjust  the 
variable  condenser  toward  the  180°  end  of  the  scale,  that  is,  for 


RADIOTELEGRAPHY.  125 

larger  values  of  capacity  and  for  longer  wave  lengths  than  at  reso- 
nance until  the  wattmeter  reading  is  again  reduced  to  one-half  its 

I  2 
value  at  resonance.     Note  this  reading   -^-  =  I22  =  I12  and  the  con- 

£l 

denser  capacity  C2.  From  the  readings  taken  at  resonance  and  on 
both  sides  of  resonance,  the  following  formulas  can  be  used  to  de- 
termine the  desired  decrement,  in  which  ^  and  d2  are,  respectively, 
the  logarithmic  decrements  of  the  wave  meter  and  the  circuit  under 
measurement;  7r  =  3.1416;  CE  is  the  capacity  of  the  condenser  in 
microfarads  or  other  convenient  units,  where  resonance  was  ob- 
tained, and  Ct  is  the  capacity,  where  the  wattmeter  current  was  re- 
duced to  one-half  its  value  at  resonance  on  the  short-wave  length 
side  of  resonance,  and  C2  is  the  corresponding  capacity  on  the  long- 
wave length  side.  The  formula  as  usually  written  gives  the  sum 
of  the  two  decrements,  from  which  the  decrement  of  the  wave  meter, 
which  is  given  as  a  part  of  the  calibration  of  the  instrument,  must 
be  subtracted  to  give  the  desired  decrement.  Two  measures  of  the 
decrement  can  be  obtained  from  the  above  values;  the  first  from 
the  readings  at  the  resonance  point  and  one  side  of  the  resonance 
curve,  and  the  second  from  the  resonance  point  and  the  other  side 
of  the  curve. 

For  the  capacity  at  resonance  CE  and  that  on  the  short-wave 
side  C: 


Similarly  for  the  capacity  at  resonance  CR  and  that  on  the  long-wave 
side  C2: 


As  the  resonance  curve  is  not  always  symmetrical  it  is  best  to  take  the 
average  of  these  two  values  for  the  average  value  of  the  sum  of  the 
decrements. 

Instead  of  computing  two  values  and  taking  the  average,  the  fol- 
lowing single  formula,  using  the  values  on  both  sides  of  resonance, 
gives  approximately  the  same  value  for  the  sum  of  the  decrements : 


It  will  be  noted  that  the  values  of  IB2,  It2,  and  I22,  do  not  appear  in 
the  formulas  but  rather  CR,  C^  and  C2,  which  however  depend  on  the 
relative  values  of  IR2,  It2,  and  I22. 

The  following  numerical  example  will  show  the  use  of  the  formulas, 
the  data  being  taken  from  the  resonance  curve  of  figure  84,  where,  as 
described  on  page  60,  a  single  turn  of  wire  had  been  inserted  in  the 


126  RADIOTELEGRAPH  Y. 

antenna  of  a  quenched-spark  set,  the  two  circuits  of  which  had  been 
carefully  tuned  to  resonance  as  described  on  page  58. 

Wattmeter  Wave-meter 

readings,  or  I2.  capacities  in  mf. 

0.  008  ....................................................  0.  00125 

.  Oil  .  00124 

.016  .  00123 

.  022  .  00122 

.  028  .  00121 

.  035  .  00120 

.038  .....................  Resonance  ......................  001195 

.  036  .  00119 

.  026  .  00118 

.  016  .  .  00117 

.  Oil  .  00116 

.  007  ....................................................  0.  00115 

From  the  plot  of  the  curve  in  figure  84  it  is  seen  that  at  resonance 
IB2  =  0.038CEis0.001195mf.;  and  at  V=^  C,  is  0.001175  mf.,  and 

at  I22  =       C2  is  0.001225  mf.;  hence 


^ 


Similarly, 


Average  value,  dt  +  d2  =  0.065 
Using  the  single  formula 

a.L*      i  e>7  0.001225  -0.001175     1RTvftn,1Q 
o1  +  o2  =  1.57  -  nm  in,  —     —  =  1.57x0.0418  =  0.066 

.uui  lyD 

The  value  of  dt  being  given  with  the  wave  meter  as  0.016,  it  is  seen 
that  ^2  =  0.066  —  dl  =  0.050  by  both  formulas,  which  is  the  logarithmic 
decrement  per  complete  oscillation  of  the  antenna  circuit. 

In  some  cases  it  is  convenient  to  be  able  to  use  wave  lengths  in 
meters,  instead  of  capacities  for  the  .computation  of  the  decrement- 
The  corresponding  formulas  are,  for  the  short-wave  side  of  resonance* 

*    _1_  *          o       R  1        A  OQ*R        *1 

Q1  +02  =  *R  -  J  -   =  O.^SO  -  r  - 

Xj  /! 

and  for  the  long-wave  side, 

^    _1_  ^          0—2        *R        ft  OQ*2        *R 

O1  +  o2  =  ^7r  —  »  —  =  b.^o  —  ^  — 

*2  ^2 

and  for  the  single  formula,  using  the  measures  on  both  sides  of  reso- 
nance, ^+^  =  71^^  =  3.14^^ 

^R  ^R 

in  which  AR,  Xv  and  X2  are  the  wave  lengths  in  meters  or  other  con- 
venient units  corresponding  to  the  capacities  CK,  C1}  and  C2. 

There  are  other  formulas  for  the  sum  of  the  decrements,  as  in  terms 
of  the  frequencies,  etc.,  but  as  they  are  not  in  common  use  they  will 
not  be  given  here. 

The  preceding  formulas  apply  only  in  the  case  where  IR2  and  It2  and 
IE2  and  I22  are  both  in  the  proportion  of  1  to  £.  If  for  any  reason  this 


RADIOTELEGKAPHY. 


127 


0.040 


0-030 


0.020 


0.010 


0-000 


Capacity  in  M.F. 


128  RADIOTELEGRAPH  Y. 

relation  is  not  true  the  full  formulas,  from  which  the  preceding  were 
obtained,  must  be  used  as  follows: 


-u&gU^ 

^2  *      K    —  -*- 

C.-C.   /-F~ 

"crViTEi 

-a.-»M»1kfi?ii 

Al  \    AR  Xl 


iR2-r 

In  general  in  using  these  last  six  formulas  the  complete  resonance  curve 
is  drawn  from  the  observations  as  shown  in  figure  84.  In  the  third  for- 
mula, in  which  values  on  both  sides  of  the  resonance  curve  are  used,  Ct 
and  C2  must  be  taken  from  the  curve  for  the  same  value  of  I2;  and  simi- 
larly in  the  sixth  for  /^  and  /i,  for  the  same  value  of  I2.  In  any  of  these 
formulas  if  Ix2  or  I22  is  made  £  IR2  the  expression  under  the  square-root 
sign  becomes  equal  to  1,  and  hence  the  simplified  form  previously  given. 

Sometimes  a  hot-wire  ammeter  is  furnished  with  the  wave  meter 
instead  of  a  wattmeter;  in  which  case  the  value  of  CB  is  obtained  at 
the  value  IR.  The  values  CA  and  C2  must  be  obtained  when  Ix  and  I3 
are  equal  to  0.7  IR  (more  accurately  0.707  IR).  With  these  values  of 
CR,  C1?  and  C2,  or  the  corresponding  values  of  /1R,  A1;  and  /t,,  the  sim- 
plified formulas  for  the  sum  of  the  decrements  can  be  used  as  above. 
If  the  ammeter  readings  are  taken  at  the  relative  values  of  IR  and  0.707 
IR,  the  squares  of  these  readings  are  in  the  necessary  ratio  of  1  to  J. 

Measures  of  the  logarithmic  decrement  can  also  be  made  without 
the  use  of  the  wave  meter  in  certain  special  cases.  If  a  single  circuit 
with  high-frequency  resistance  R,  inductance  L,  and  capacity  C  is  not 
coupled  with  any  circuit,  or  very  loosely  coupled  with  a  primary 
quenched-gap  circuit,  theory  shows  that  its  logarithmic  decrement  per 

T> 

complete  oscillation  can  be  computed  from  the  formula  ^=o~MT7 

where,  if  R  is  in  ohms,  L  must  be  in  henrys,  and  -N  is  the  frequency 
in  oscillations  per  second.  Thus,  if  the  antenna  whose  decrement  was 
measured  by  the  wave  meter  above  as  being  0.050  should  have  a 
resistance  of  6  ohms,  an  inductance  of  200,000  cm.  or  0.0002  henry, 
and  should  be  oscillating  at  a  frequency  of  300,000  or  a  wave  length 
of  1,000  meters,  its  decrement  by  the  above  formula  would  be 

2  X300,006OX  0.0002  or  °-050>  as  found  by  the  Wave  meter- 

GEORGE  P.  SCRIVEN, 

Brigadier  General, 
Chief  Signal  Officer  of  the  Army. 


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